Does peanut butter ever go bad after opened?

Eventually the oil goes rancid and it will taste horrible.

However I have kept and used peanut butter for a very long time past the expiration date without any sign of rancidity. On the other hand I have bought some "no name" brands of peanut butter that already tasted rancid when I first opened them the day that I bought them...

However rancid peanut butter will not make you sick.

There are some molds and other microorganisms, some of which are toxic and will make you sick, that grow on peanuts and will also grow in peanut butter.

So yes peanut butter can go bad, but I eat lots of it and never encountered a problem (other than rancidity in "fresh" peanut butter, as I said above).