Most states allow use of force and even deadly force to protect a third party; HOWEVER, there are serious practical problems with doing so. A couple of common ones are:
1. The person you are trying to "save" may not always appreciate it. Domestic violence between partners/relatives can often change into both beater and beatee attacking whoever intervenes.
2. You don't know the whole story of what is going on:
http://thefiringline.com/forums/showthread.php?t=454222
In the above link, a man uses deadly force in defense of a third party and ends up killing an off-duty police officer - and one he was acquainted with even. Turns out that despite being present and in close proximity during the entire scenario, he didn't fully grasp what was going on.
This website also has an excellent discussion of the problems surrounding the question you are asking:
http://corneredcat.com/Saving_the_Life_of_a_Stranger/
It is worth a read.