A good person turning into a billionare by accident would donate all his extra money away as soon as he gets them. There aren't any good billionares.

So if Warren wants to do the morally correct thing why doesn’t he hire assassins to kill the bad billionaires?

Most people prefer to stay out of prison.

Technically, we don't know that he isn't. That kind of money likely means an assassination would be heavily planned out and need to look like natural causes because if was obvious he'd potentially put a target on his own back from other billionaires.

But I seriously doubt he'd do this anyway.