The Colorado STEM shooters never had a motive released. Not sure where you got that from. One of the shooters was literally registered Democrat for less than a year. The other wasn't even old enough to vote. There's some suspected bullying that went on but that's about all anyone knows so again not a "left wing" shooting.
The only one on your list that's actually verifiably a left wing violent act is the guy who shot at Republicans. That's absolutely verified.
Contrast that with the church and synagogue shootings where people literally admitted that they were on the right. Contrast that with the pizza shop incident where Hillary and left wing people were supposed to be hiding slaves in the basement of a building with no basement, which was circulated by the right. Literally bombs sent to Democrats by people on the right. Finally the President himself calling for his supporters to beat people up at rallies.
The only way one denies the violence is more common on the right is if one is predisposed to believe it. Are there some cases where it's unclear why violence happened that have been misattributed to the right? Sure how could there not be, but there's also overwhelming evidence of violence on the right.