History

Taken as an obvious parallel with the attempted assassination of Trump, political violence in America is very clearly a bad thing at this point.

But trying to stop looming fascism through random political violence is like trying to stop a bear attack by covering yourself in steak sauce.

In my opinion, the issue isn't that it was political violence, but it was (as you said) random political violence. Thoughtless and extreme political violence. What happens if Trump was assassinated? I propose that Trump would be replaced by a more competent career politician, more appealing to capital and to the closer of the Democrats. I think Trump is like Hitler in that they're somewhat of outsiders to politics who understand popularity more than pragmatism, related to that first answer you gave. There's no point to just cutting off the head of the hydra, the failed tactic of propaganda of the deed already demonstrated that after killing a lot of presidents, kings and police chiefs. The problem with the Republican Party is systematic, not that it's being headed by Trump. (obviously this all may not be the worldview of the would-be assassin, I'm just explaining this from a non-Republican perspective)

But political violence as a broad umbrella goes well beyond this example and there's not really any reason to leave it until it's too late. I think it's perfectly appropriate to have a Battle of Cable Street before the fascists get elected, and it (along with later '43 Group violence) worked. The BUF deteriorated due to forceful repression by ex-military antifascists. These are not just random acts of violence, but intentional, tactical violent resistance and then violent suppression of fascist movements. Non-violent methods are generally preferred by anti-fascists today for good reasons (easier to get mass involvement, more sustainable, etc.), but violent actions are a legitimate and effective part of the arsenal against fascism when used appropriately. Attempting an assassination of a state figurehead is not that.