in Fallout, everything must die and i must steal, loot everything after they become of no use

Yup, you can play like that, just watch your bounty.

Or kill all witnesses and you wont get any bounty

Not the best way, but yeah.

Word is on the street is that the authority has agents in the local wildlife; you never know which animals will actually rat on you if they witness it.

Yes and No. Yes because there's a lot of people you can randomly kill. No because some people you can't kill so if your in a town with an immortal npc and try to kill everyone, he will keep coming back to fight you, Looting and stealing you can do.