I don't think that you're taking into account the context of this passage; God specifically told Onan that he should take his late brother's wife as his own and bear children with her in his brother's name. He was punished not because he spilt his semen on the floor but because be disobeyed God. It's not in any reference to masterbation.
It is a sin to make fun of anyone. I believe that this passage is very important for Christians as it shows us that children- of a certain age- are not immune from sin and thus children are not immune from eternal damnation. It is vital that we teach our children right from wrong as early as possible.
Nevertheless, what Elisha did was greatly condemned by God and it prevented him from entering heaven after his death. In fact, this is a really good example of why Christians should never take matters into their own hands when it comes to justice. Elisha deemed it okay to punish these children through the power that God gave him, which angered God as he had no right to judge them. This passage should be taught repeatedly to those militant Christians who enact violence on people who disagree with them.
Slavery has been practiced since the beginning of time. The slavery practiced by those mentioned in the Bible wouldn't have been as brutal and inhumane as that of the Trans-Atlantic Slave Trade or the Arab Slave Trade. On a side note, many black people also owned slaves in Africa so I don't really understand your point.
I hope I've been helpful

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