A few questions about God and Calvinism

Hi everyone, I hope you're having a great day.

(Reading my post before posting, I've realized it looks a bit "agressive" but I assure you it's not, I just want to learn more about Christianism from Christian people :) )

Im an atheist but I've been reading a lot recently about the Protestant Reformation in my country (France) and I have to admit that I learned to admire the Huguenots that were slaughtered by the Catholics. I decided to star learning new things about Christianism so I started to read some books, and a bit a of Bible (which is really hard to read tbh).

Now, I've got two questions for you that are fairly different :

I don't understand how God can change suddenly between the old and the new testament. From what I've read, the God in the old testament is not acting like a God; he's slaughtering lots of people, he is arbitrary, really non-Godlike. Why would I want to worship him ? And how do you explain that he suddenly sent his son on earth to save us all ? What about Lucifer ? Do we know why he rebelled against God ? From what I've read, all Lucifer has done is try to rebel against a tyrant God that nobody would want to worship.

Now second big question. What is the election ? I've read that it's a concept from Calvin. From what I understood, God has chosen some people (reminds me of the jewish people) that will go to paradise and other that will go to hell. How did he chose ? If I'm a good person during all my life but God has decided that I'm going to hell, there's noting I can do ? Does this mean that I have no free will ?

Thanks for you time friends, G.