I am happy that all sentences are correct. But now I wonder if they all have same meanings.To my ear, of course, they all mean the same. Could you explain them to me?
I know it's difficult to explain them, but you can reply to me when you get free time. I will wait for it until then.
I don't like to contradict others, but I don't agree that these are all okay. Grammatically, yes, but that doesn't mean they are natural or make sense.
The crocodile hunts by instinct is okay, but not your first sentence.
Your second sentence is okay - it means that he just has a mean, nasty disposition and he doesn't do things that are nice.