He was more of an enthusiast. To link him to political violence would require a couple of things - a manifesto of some kind, or him targeting the people that antifa, Warren, or Sanders are going after in their speeches. Instead of going after bankers, political donors, or some place frequented by upper classes, he went after a bar and restaurant district and one of his victims was his own sister.
If a Trumper shoots up his workplace after getting fired it probably isn't political - it's still a tragedy though.
But if that same person goes after the targets of political rhetoric then it is probably political in nature.
In other words, the Dayton shooting doesn't meet the base requirements for being a left wing attack, despite the perpetrator being a left winger himself. The history the guy has shows warning signs of violence going back before he was likely to have formed his political identity to begin with.