Many people who do things like this are just crazy. Or they're on drugs. Others have personal grievances.
They are not political. The insane ones are just too far separated from the usual political categories and don't fit into the usual political spectrum.
But I get it. Whenever something like this happens, people rush to say that they don't have anything in common with the killer and that their political opponents do.
In any case, Breivik the Norwegian mass murder, wasn't what most people would call a leftist. You might call him that, but the people he was killing were those he saw as leftists and he killed them for that reason.
Breivik the Norwegian also offered his killing services to Islamonazis. He said that they wouldn’t be looking for a white person such as himself. He wasn’t against Islamonazism. He just didn’t want them in his country.
Breivik was a neo-Nazi, which is fundamentally Leftist. This confuses a lot of people because Nazism is so different from the Leftists we typically encounter, but Nazis view the government as a positive force and believe in enforcing equality (if you’re the correct race); both of these are Leftist traits.
Conservatism also believes in adherence to the law (except for extraordinary circumstances, such as during the American Revolution), while mass murderers fundamentally do not.
Mass murder (when done to people not known personally, and planned out) is fundamentally done by Leftists. I will concede that a guy attacking people while on bath salts may not be thinking of Leftist political ideals at the moment, but it was Leftist thoughts which brought him to that state.