You honestly don't need many reasons at all to kill conservatives, when you look at the last 100 years of Conservatives holding society back and stifling progress, It's morality and human decency that is preventing people from slaughtering conservatives wholesale.
tl;dr
You have to invent wacky and insane reasons and absolutely fucking retarded conspiracies to kill Liberals.
A guy who wants.. Everyone to have free health care? Gotta stop him, he's really an evil agent trying to poison you with 4G and the Gay Gene. The only way to stop him is to kill him!
You have to invent wacky and insane reasons to not kill Conservatives.
A guy who wants women to be broodmares , feels that you should own nothing ,be happy, be his slave and proudly professes that gay people should be stoned to death? It's not nice to kill people for their opinion.