How did your heroic poll watchers determine who was supporting rebellion?
The same way they would determine a bank robbery, I guess, suspicious-acting people. It looks like they were meant to intimidate the fraudulant voters. As the bible says, law-abiding people have no reason to fear His sanctioned powers that be, but His sanctioned powers that be are a terror to the law-breaking. No one was shot for voting McClellan so your theory is ridiculous, it's clear they were after the arsonists.