I'm not saying that you are defending the alignment that "best" suits this power, but, can you see that it is even more risky in the hands of a townie?
Townie: Bob fights Jack. Nobody but Bob, and alignment cops know what Bob is. Nobody but Jack and alignment cops know what Jack is. Both statistically have equal chances of being town, and equal chances of being voted/lynched.
Scumbag: Bob fights Jack. Nobody but Bob, and the rest of the mafia, and alignment cops know what Bob is. Nobody but Jack and alignment cops know what Jack is. There is only a chance that Jack is town. While both players still statistically have the same odds of being voted/lynched, the inherent balancing by giving this power to mafia makes sense.
Neutral: Bob fights Jack. Bob only cares about getting a kill, so he doesn't care what alignment Jack is. Otherwise, equal to the scumbag scenario. Again, inherently balanced.
Granted, I've never setup a Forum Mafia game, but, if it counts for anything, I have "moderated" for AD&D for many years, and balance/fairness is always a priority to make the game fun.
I'm not saying you are wrong, and I'm not saying I am right. I just want you to see that there are other possibilities.