Jonah 4:1
But it displeased Jonah exceedingly, and he became angry.

Jonah became really upset at God for several reasons.
First, because he knew something like this might happen.
Jonah did not like the Assyrians. They tortured, killed, raped, ravished, and conquered as many as they could. Getting rid of them was not a big deal to Jonah. He was glad for it to happen but God relented from their destruction because of their repentance.
Second, he was mad because God made him turn around and make the trip to Nineveh. He was halfway to the sunny south of Spain when God and a great fish interrupted the trip.
Third, Jonah is upset because God is so gracious, merciful, slow to anger, and abundant in lovingkindness.
He was so angry that he’d rather die than live in a world with the Ninevites.
I can’t think of any legitimate reason to be angry when sinners repent.
The angels in heaven aren’t angry. They rejoice over one sinner that repents.
If the angels do it, shouldn’t we?
Rejoice with me, for I have found my sheep which was lost!’ I say to you that likewise there will be more joy in heaven over one sinner who repents than over ninety-nine just persons who need no repentance. Luke 15:6-7