Nope. You aren't responsible for another persons actions. And believe you me, there is no end of issues that can make a good person be mean. Maybe they've got something rotten in them, maybe it's a temporary thing, or a misunderstanding, a failure of communication. Or maybe they're just plain awful and really good at hiding it. It doesn't mean that you are flawed in some way because you don't know exactly what somebody is going to do in 5 years.
Nobody can do that. Nobody can see the future of even 5 minutes, much less 5 years.
And like others have said, people change.. Maybe they were the best person ever, but something made them turn bad. I, for instance, I'm mean. I have issues that stretch back 20 years that sometimes make me behave horribly to people. I try as hard as I can not to let them, but sometimes I fail. It's just a human thing.