They're not wrong. In some respects, the identity and motivations of the assassin are irrelevant. There are many Americans who hated Kirk with a passion and wanted him dead - probably in every city in the country. Many of them are gloating that he was killed. I've even seen acquaintances in other countries sharing the same sentiments on social media. If the shooter wasn't in fact a "leftist", any response (crackdown, increase in political violence) based on that premise will only be a natural progression of politics as they currently are - in which "leftists" have been committing similar crimes, albeit not as high-profile.
In the absence of something like ponerology, which doesn't have enough political capital to be a live option at the current moment, the natural response is violence or force: "they need to be crushed." This can take the form of a law enforcement crackdown, or, in geopolitics, war (like Russia in Ukraine). That's because debating doesn't work, ultimately, on the portion of the population who pose the greatest threat.
It could just as easily be that the assassin is an actual transgender lefty Hamas supporter who has funding that has been made to appear to be from Iran. Mossad, or any other group with similar motives and capabilities, could be doing the same thing with dozens or hundreds of such types, creating suitable paper trails.
