I catch myself thinking this way sometimes, when I feel hatred towards a racist.
>"Feeling this way is fine... finding the spot where social control of antisocial behavior morphs into antisocial behaviour itself is the hard and important part."
I'd use the term "response" rather than "control". For me it depends on the person that is doing bad things. When it's a racist, it's likely something that is a deeply rooted defect in the person and I just don't have the patience (or the skill) to deal with people like that.
To help with the feeling part, sometimes I just think to myself that that person has a mental illness and that's the way it is. But you don't just let them continue to harm others, you need to respond.
Online communities make it very easy to exclude someone simply with a click of the mouse. IRL it's a little more difficult, although it still happens. It used to be that an "exclusive" community was more highly valued socially -- high society, exclusive clubs, etc. But I think the trend now is moving away from that attitude, mainly because historically the exclusivity criterion was often based on race, sex or disability.
I think banning someone from a group should be a last resort, only after other measures have failed. (The exception being people who are intentionally trying to disrupt and preventing others from freely participating, like with a DOS attack or malware, etc.)
Also, I think that excluding someone for their behavior, for breaking the rules of the group is much different than excluding someone because of who they are, due to something beyond their control. And excluding someone based on their opinions should be avoided.
IRL = in real life