True. But to be fair, it's a pretty common thing in movies. If you're gonna have a bad guy, either they start off good or maybe neutral, or they're bad to begin with. I think it's worse when some character is introduced who's obviously the bad guy, because then you're basically waiting for him to pull some shit about 3/4 of the way through, when everything seems to be going fine. Then he does something dastardly and you say (sarcastically) "Oh, I didn't see that coming". On the other hand, that's still better than someone who's been a good guy the whole way suddenly turning 180 because of something having to do with his mom/dad/brother/whoever who was never even mentioned before. That's just cheap.
And in the case of Lotso, it was actually a double-twist or something. They'd saved his ass, he seemed grateful, and I actually thought he was gonna turn out okay, like he'd seen the light or something, and then he still left them to die. It was pure spite at that point; he didn't even have a reason other than he was an asshole. Stinky Pete at least had something like a rationale for wanting to stay in the box and retain his collector's value.