We think a lot of things might be evil, but we don't go around killing others to change them. That is something we use the process for - change. I thought clinton was evil, but instead of becoming violent towards him I voted against him.
The problem we have, and I DO see your point BTW (one I have made myself many times), is that the government allows something which we find totally wrong - IE we are not fighting a PERSON who is abusing a child (ie killing it by abortion), we are fighting the entire society and legal structure.
If I saw someone trying to kill their kid, I would be able to use lethal force if needed to save the life of the child - and I would be backed up by society and the legal system. Killing the doctors, blowing up the clinics, etc will in the end prove fruitless. Changing the beliefs of people so they agree with you and change the law would in the end save more lives more quickly.
That all said I will say that if the average american say someone, like the tape we all saw of the woman beating her kid, trying to kill a child they would feel justified in using force to stop them (even if the law allowed us to kill kids). We don't with abortion because a) we don't see it as it happens and b)we don't witness the victim's face, hear their yells, etc.
Out of site, out of mind - and most people don't think it is worth their freedom to save the life of someone they do not witness being killed. If all abortions were done live on big screens in stadiums I guarantee people would rush out to stop them.