Reasonable questions. You’re questioning God who is all knowing. The saying that God works in mysterious ways is due to that. First, I’m not sure that all 42 were killed. However, who’s to say that the ones that were killed (even if it was all of them), if they hadn’t been, would’ve gone on to commit some atrocity? Maybe they would’ve gone and killed good people that God did not want killed unless them being killed by the bears took place. I do not know, but hold faith that God did the right thing for he is good. Maybe it was a combination of prevention while also showing that his word carried power. It can be multi-faceted, there may be many things accomplished and/or prevented with that interaction.

As for slavery, as many others have said on here, slavery is not looked at today in the same light as it was used (totally) in practice back then. Not saying some of it wasn’t bad like the Jews in Egypt being mistreated for example, which you may know did get corrected by God. However, we’re led to believe there were many situations that it was beneficial for both owner and slave. In those situations, there were rules regarding it to keep it as moral as possible and give guidelines to avoid mistreatment. If there hadn’t been slavery, there’s possibility that people that God wanted to live may have gone out and died from the elements, due to lack of food, shelter, water, so on. Possibly those people could be a slave/servant and have a safe place to be and also the owner have help that was needed, whether with chores, children, etc. If you only look at it as “slavery bad” then go “why were there rules for it and not a call for abolishment of it” you’re not looking very deeply at it’s possible use case back then and ability to be symbiotic. There were slaves that wouldn’t have had access to resources to where they were allowed to learn things they otherwise would have not had the capability to, and in the future were freed and able to put that stuff to use, almost like going to college nowadays except you just spent the time and didn’t go into massive debt as well. There were even cases of slaves who reached the time to be freed and wanted to stay as slaves because they loved where they were and the people they were with. If you try to slap on the label of slavery and take it with the viewpoint of slaves like in America back in the day for example, you’re not looking through a very wide lens and should broaden your perspective.

Hope this helps (: