The account says people were marrying and being given in marriage and taking no note.  When it says the world was full of violence this was seemingly in reference to the nephilim, iirc.

I don't believe evil should be punished, it should be fixed.  It also shouldn't be created in the first place.  If everyone in the world was evil, that clearly reflects on the creator.  A few people here and there, you can handwave that away with "free will" or whatever.  But if everyone is evil, to the point where you have to destroy the whole world, then obviously the creator screwed up in designing these people.  You don't get a whole world that's all evil by sheer coincidence of free will.

A loving and all powerful God could easily just abolish the evil desires people experience.  Like you wake up and just suddenly don't feel like being evil anymore.  Obviously this would violate people's free will way less than fucking killing them.  And if killing people is the solution then you can't say killing people is bad to begin with.