Just a guess, but my thinking is a townie shot isocialism. Then the question is if the Mafia would still have both their bullets or if one was wasted on somebody who commuted.
What makes you think Isoc was killed by a townie?
Math
I don't see it, just because there are more town than scum doesn't mean Isoc was killed by town.
Isoc is a strong player, I think this was a good decision by scum to take him out.