1. Be slow to anger because God is slow to anger:
Exodus 34:6: The LORD passed before him & proclaimed, "The LORD, the LORD, a God merciful and gracious, slow to anger, and abounding in steadfast love and faithfulness"
2. Be slow to anger because God knows better than you do:
Job 38:1-4, 8-12: Then the LORD answered Job out of the whirlwind and said: "Who is this that darkens counsel by words without knowledge? Dress for action like a man; I will question you, and you make it known to me. "Where were you when I laid the foundation of the earth? Tell me, if you have understanding . . . Or who shut in the sea with doors when it burst out from the womb, when I made clouds its garment and thick darkness its swaddling band, and prescribed limits for it and set bars and doors, and said, 'Thus far shall you come, and no farther, and here shall your proud waves be stayed'? "Have you commanded the morning since your days began, and caused the dawn to know its place?"
3. Be slow to anger because God will be angry with you:
2 Kings 17:11-12: And they did wicked things, provoking the LORD to anger, and they served idols, of which the LORD had said to them, "You shall not do this."
4. Be slow to anger because sinful anger results from a failure to trust God:
Psalm 37:1-8: Fret not yourself because of evildoers; be not envious of wrongdoers! For they will soon fade like the grass and wither like the green herb. Trust in the LORD, and do good; dwell in the land and befriend faithfulness. Delight yourself in the LORD, and he will give you the desires of your heart. Commit your way to the LORD; trust in him, and he will act. He will bring forth your righteousness as the light, and your justice as the noonday. Be still before the LORD and wait patiently for him; fret not yourself over the one who prospers in his way, over the man who carries out evil devices! Refrain from anger, and forsake wrath! Fret not yourself; it tends only to evil.
5. Be slow to anger because being slow to anger is wise:
Proverbs 14:29: Whoever is slow to anger has great understanding, but he who has a hasty temper exalts folly.
6. Be slow to anger because being slow to anger encourages peace:
Proverbs 15:18: A hot-tempered man stirs up strife, but he who is slow to anger quiets contention.
7. Be slow to anger because anger often leads to sin:
Proverbs 29:22: A man of wrath stirs up strife, and one given to anger causes much transgression.
8. Be slow to anger because sinful anger is foolish:
Ecclesiastes 7:9: Be not quick in your spirit to become angry, for anger lodges in the bosom of fools.
9. Be slow to anger because sinful anger is one of the works of the flesh that leads to hell:
Galatians 5:19-21: Now the works of the flesh are evident: sexual immorality, impurity, sensuality, idolatry, sorcery, enmity, strife, jealousy, fits of anger, rivalries, dissensions, divisions, envy, drunkenness, orgies, and things like these. I warn you, as I warned you before, that those who do such things will not inherit the kingdom of God.
10. Be slow to anger because sinful anger gives opportunity to the devil:
Ephesians 4:26-27: Be angry and do not sin; do not let the sun go down on your anger, and give no opportunity to the devil.
Ligon Duncan: "Paul knows that anger is hard to handle responsibly, and so the evil one lurks, ready to exploit us, ready to provoke us, ready to prompt us, ready to sail with the wind. Our sinful anger is kindled, he is ready to fan the flame, he's ready to blow wind into those sails and help us go along in the expression of that sinful anger, because the devil wants to use our sinful, prideful, selfish anger to breach the fellowship of the body and rob God of glory. And he wants to use it to destroy your souls. And when he can do all three of those things at the same time, he loves it! When he gets to rob God of glory, destroy the fellowship of the body, and send your soul on the way to destruction, it's three for the price of one for him! He loves that! And the Apostle Paul says 'Friends, understand that your battle with anger is a matter of spiritual warfare, because when you are expressing sinful anger, the evil one is at your shoulder ready to do his [wicked] work.'" (https://ligonduncan.com/anger-and-the-glory-of-god-469/)
11. Be slow to anger because sinful anger grieves the Holy Spirit of God:
Ephesians 4:30-32: And do not grieve the Holy Spirit of God, by whom you were sealed for the day of redemption. Let all bitterness and wrath and anger and clamor and slander be put away from you, along with all malice. Be kind to one another, tenderhearted, forgiving one another, as God in Christ forgave you.
12. Be slow to anger because sinning in anger is not who you are in Christ:
Colossians 3:1-13: If then you have been raised with Christ, seek the things that are above, where Christ is, seated at the right hand of God. Set your minds on things that are above, not on things that are on earth. For you have died, and your life is hidden with Christ in God. When Christ who is your life appears, then you also will appear with him in glory. Put to death therefore what is earthly in you: sexual immorality, impurity, passion, evil desire, and covetousness, which is idolatry. On account of these the wrath of God is coming. In these you too once walked, when you were living in them. But now you must put them all away: anger, wrath, malice, slander, and obscene talk from your mouth. Do not lie to one another, seeing that you have put off the old self with its practices and have put on the new self, which is being renewed in knowledge after the image of its creator. Here there is not Greek and Jew, circumcised and uncircumcised, barbarian, Scythian, slave, free; but Christ is all, and in all. Put on then, as God's chosen ones, holy and beloved, compassion, kindness, humility, meekness, and patience, bearing with one another and, if one has a complaint against another, forgiving each other; as the Lord has forgiven you, so you also must forgive.
13. Be slow to anger because prayer should be done without sinful anger:
1 Timothy 2:8: I desire then that in every place the men should pray, lifting holy hands without anger or quarreling
14. Be slow to anger because the anger of man does not produce the righteousness that God requires:
James 1:19-20: Know this, my beloved brothers: let every person be quick to hear, slow to speak, slow to anger; for the anger of man does not produce the righteousness that God requires.
15. Be slow to anger because vengeance belongs to God:
Romans 12:16-21: Live in harmony with one another. Do not be haughty, but associate with the lowly. Never be conceited. Repay no one evil for evil, but give thought to do what is honorable in the sight of all. If possible, so far as it depends on you, live peaceably with all. Beloved, never avenge yourselves, but leave it to the wrath of God, for it is written, "Vengeance is mine, I will repay, says the Lord." To the contrary, "if your enemy is hungry, feed him; if he is thirsty, give him something to drink; for by so doing you will heap burning coals on his head." Do not be overcome by evil, but overcome evil with good.
16. Be slow to anger because sinful anger leads to murder:
Genesis 4:3-8: In the course of time Cain brought to the LORD an offering of the fruit of the ground, and Abel also brought of the firstborn of his flock and of their fat portions. And the LORD had regard for Abel and his offering, but for Cain and his offering he had no regard. So Cain was very angry, and his face fell. The LORD said to Cain, "Why are you angry, and why has your face fallen?If you do well, will you not be accepted? And if you do not do well, sin is crouching at the door. Its desire is for you, but you must rule over it." Cain spoke to Abel his brother. And when they were in the field, Cain rose up against his brother Abel and killed him.
Matthew 5:21-22: You have heard that it was said to those of old, 'You shall not murder; & whoever murders will be liable to judgment.' But I say to you that everyone who is angry with his brother will be liable to judgment; whoever insults his brother will be liable to the council; & whoever says, 'You fool!' will be liable to the hell of fire.
17. Be slow to anger because sinful anger will destroy you:
Frederick Buechner: Of the seven deadly sins, anger is possibly the most fun. To lick your wounds, to smack your lips over grievances long past, to roll over your tongue the prospect of bitter confrontations still to come, to savor to the last toothsome morsel both the pain you are given & the pain you are giving back . . . in many ways, it is a feast fit for a king. The chief drawback is that what you are wolfing down is yourself. The skeleton at that feast is you.
18. Be slow to anger because Jesus Christ never got sinfully angry and then died for sinners and rose from the dead to save sinners from all of their sinful anger forever:
Romans 5:8: but God shows his love for us in that while we were still sinners, Christ died for us.