The LORD said to Moses, "Take all the leaders of these people, kill them and expose them in broad daylight before the LORD, so that the LORD's fierce anger may turn away from Israel."
The LORD's anger burned against Israel and he made them wander in the desert forty years, until the whole generation of those who had done evil in his sight was gone.
Through your own fault you will lose the inheritance I gave you. I will enslave you to your enemies in a land you do not know, for you have kindled my anger, and it will burn forever.
For his anger lasts only a moment, but his favor lasts a lifetime; weeping may remain for a night, but rejoicing comes in the morning.
Psalm 103:8 (also Exodus 34:6-7)
The LORD is compassionate and gracious, slow to anger, abounding in love.
Wrongdoing angers God. If someone does wrong and does not repent of it, God will be angry over their original sin and their continued sin of refusing to admit their wrong. However, as soon as someone genuinely seeks God's forgiveness, God forgives them and his anger ceases. For instance, the majority of Jerusalem in Jeremiah's time remained unrepentant, thus God continued to be angry with them. Yet if they had repented and asked forgiveness, God would have forgiven them and not been angry with them any more. In fact, in the following chapter in Jeremiah, God says:
If at any time I announce that a nation or kingdom is to be uprooted, torn down and destroyed, and if that nation I warned repents of its evil, then I will relent and not inflict on it the disaster I had planned. (Jer 18:7-8)