I think it's the exceptionally kind people who become targets.
If your behavior is unusual, meaning further from the normal variance of generally observed behavior, it becomes a precedent that you are probably flawed and don't know what you're doing.
The assumption as to why you behave this way is that you're naive. You want to live in utopia, can't recognize that this is not it, and fool yourself into thinking that this is it and people by nature are very good (which they might be but that's a different question). The observation is that you lack the normal competitive drive and instinct of self preservation because you're too dumb to compete and are too vaguely aware or reality to know the imperative of self preservation.
What about people on the other end of the spectrum. One might assume that the guy on death row automatically has no emotion, has good general intelligence but got caught, luckily, because he is very risk seeking.
Sometimes the stereotype is observed true, sometimes false, but is still a stereotype meaning it has some kind of logical or observational precedent yet is by definition (I say that often) still a generalization.