There are claims by some conservatives that people are unfairly overlooking the fact that the Dayton shooter was a leftist, while focusing on the anti-immigrant/nationalist motives of the El Paso shooter. A few points on this:
The Dayton shooter's (now-deleted) social media accounts do indeed indicate he was a leftist (e.g. - he endorsed socialism, said he supported Elizabeth Warren, and so on).
BUT there is to this point zero evidence that politics was the motive for his attack. By contrast, the El Paso killer wrote a manifesto that leaves no doubt about the nature of his motives. Both left-wingers and right-wingers sometimes kill for non-political reasons. Indeed, such murders are much more common than politically motivated ones.
The fact that one of the dead was his sister is some evidence for the idea that the motive was personal. Contrary to the comment by the police chief quoted below, it is indeed entirely believable that the killer might target his sister. A large percentage of murder victims are in fact killed by members of their own families. All too often, people inclined to violence have especially strong grievances against those closest to them.
Perhaps evidence will emerge showing that this guy was indeed politically motivated. If so, I will be happy to revise my view. While there have been fewer left-wing than right-wing acts of terrorism in the US over the last 10-15 years, the former are far from completely absent, and there is definitely a history of left-wing terrorism in both the US and elsewhere. To this point, however, there is no reason to believe the Dayton case qualifies as such.
Sometimes mass shooters have personal, non-political motives. And sometimes their motives are just simply unknowable (as in the Las Vegas case in 2017). The Dayton case, so far, seems likely to fall into one of these categories. That doesn't make it any less awful. But it does suggest it has little or no connection to the perpetrator's political views.