God is a Just God and punishes sin
Just because he says he is just doesn't mean he is. Homosexual relations are not worthy of punishment. If two people who consent on doing them do them then they are not harming anybody. You have no right to try and stop them from doing that because it does not affect you or anyone other than themselves.
Doesn't evil deserve to be punished?
How is not believing in something evil? God has given no clear sign that he exist so if he does it is only his fault if people do not believe in him.
Slavery back then was different then the 1800s plantation slavery
I know but regardless it is still taking belonging of another human being against their will
and still immoral.
I'm a teenager and I have no problem with that verse
Well then there is clearly something wrong with you. Children (or even anyone) never deserve to be killed apart from if they have done something seriously bad like killing some one themselves. Especially not in such an inhumane way.