There is a distinction, I believe, between a loner and an introvert. Introverts aren't necessarily loners. They can be found within groups -- they just are not competing for attention, or at the center of action, although may contribute a lot if an extrovert makes an effort to engage them. And we used to pride ourselves in the strong, silent type. Our movie heroes, as in High Noon, were strong, silent types. We even famously elected one President. When Coolidge was told "They bet me I couldn't get you to say 3 words", his response was "you lose". Imagine that from a politician today.
Somewhere along the line we went from honoring the strong, silent hero to stigmatizing the introvert. Susan Cain, in her excellent book Quiet, points out that the concept of inferiority complex as an explanation never even existed before the 1950s when we began worshiping extroverts.
Loners, on the other hand, tend to exhibit anti-social behavior. It is more than just not wanting to be the center of the party -- they want nothing to do with other people. But point taken: it could be a warning sign of potential sociopathic behavior but it, in itself should not be a reason to stigmatize the person.
