You talk about demonstrating the seriousness of sin and also learning, but even if Jesus changed the law, it was God who created the law in the first place and does it not show the qualities of God? That he is capable of going to very high extremes and at one time expected use to do the same. I don't think what happens in the present changes what happened in the past. However, you may choose to respect Jesus for this, at least this version of Jesus, taking into account not all Christians see Jesus as merciful, but instead as every bit wrathful as his father, but I am more inclined to think Jesus was more peaceful than wrathful, at least based on my readings. God on the other hand, I don't.
Unfortunately, I don't think Jesus was there to change the law (of course it would be contradictory to some of his behaviour)....if I remember I will dig out the quote. I am pretty sure it says somewhere that basically the heaven will end before a single letter of the law would change. Though iirc there are slight variations dependent on the translation.
But, I'm not sure why God would need to learn that death as a deterrent doesn't work through the Old Testament (at least this is what you seem to be implying) because are we not led to believe that he is perfect and all knowing? He created us, surely he would know how our minds work. We've taken years of studying psychology to understand how human minds work and there's many things we understand about how humans react to negative reinforcement, but according to belief He created those minds, even if he did give them free will.
As for assuming God did evil, I am only relying on the information provided, that is correct. Given previous teachings and things I've read from within the bible, I would not say it's unlike God to show his wrathful side and not necessarily as a last resort. The person who wrote the bible didn't decide to leave any extra details in, they made it about God punishing people for insulting a prophet and not about hooligans getting their just desserts for a crime spree of actions to warrant death (maybe mass murderers). This is not unlike people getting wrathful over insulting the prophet Mohammad. It could well be like that, given how the writer has chosen to tell this story, I'd say it's more likely than these people being particularly 'evil'. Of course, you can just add extra 'possibilities' to just about any scenario because not every single piece of detail is left in, however, I would say it's safe to assume the bits left in are of significance and are there to best represent the situations. That is, if the bible is the inspired word of a perfect God. If it's the word of men, it'll be about how they want to present the story or how they remember the story.
But I am glad you agree the punishments are extreme and I am glad that many Christians believe in the peaceful teachings of Jesus and not the wrathful teachings of their God, unfortunately the same cannot be said about all and it certainly hasn't been in the past.