It is one thing to be aware of troubling people but it is quite another to label them and tag them with a "beware" sign. That immediately sets us apart from other people by virtue of our self righteousness. It says we are better than them. Out goes equanimity. Out goes Compassion.
The reality is that a sociopath is such because of his or her karma. Just like we may be self righteous by our own karma. I feel that the key is to look beyond the person and see their karma at work and to understand them from that angle.
You have made good points there. Yes, we are no different from sociopaths, in terms of wanting happiness and not wanting suffering. It is not "fair" to label them as "evil", just like how we can't say lions are evil because cute wriggly piglets are their diet. We are controlled by our karma until we attain enlightenment.
In fact there are many sociopaths who realise the inherent problems of their personality disorder and don't know how to deal with it. They can't help it and it is saddening. Here's a read of a self-diagnosed sociopath asking for help from a doctor:
http://askdrrobert.dr-robert.com/sociopath.html I suppose the reaction towards sociopaths with "beware" or "stay away" need not necessarily always be based on self righteousness / bias. Warning signs on something "dangerous" are usually based on experience and wisdom or even compassion. Like how at train stations there is a sign to say "Beware of the gap". It is not that the gap that can hurt you, its your carelessness that would cause you to get hurt should you fall into the gap. If you know the pain that sticking a finger into an electric socket can cause, wouldn't you warn others to not put their finger in the socket?
As Dharma practitioners, we strive to be compassionate with anyone and try to help everyone. But are our minds strong enough to really help a mind of a sociopath? If you read about this condition more, the mind of a sociopath is pretty complex and unique... Sometimes staying away from such minds is actually being kind to them, as they would have less opportunity to hurt another person and thus create more bad karma for themselves. I definitely would not profess that I am skilful or mentally/emotionally strong enough to help a someone who is a sociopath. So to me, it is not the sociopath's condition which limits us from helping them, but our own state and strength of mind.
So yes, lets practice compassion, but together with wisdom.