But, the US didn't participate in it.
Depends on what you mean by 'didn't participate' I suppose.
If so, it would probably be considered an attempt to remove evil dictators
Except, of course, that Allende was not a dictator but was in fact a democratically elected official and head of government, in a state which had a democratic parliamentary style constitutional government which had been in place for literally decades. Really, that's no more sensible than calling Lyndon Johnson or John Kennedy an evil dictator.
and would be seen positively at the time.
I dunno about that. This, after all, was in the late era of the Vietnam War, and one of the key blunders there had been the mucking about with the overthrow of Diem. That whole 'regime change as we see fit' had fallen into a lot of disfavour.
Later on, people would start blaming Nixon for Pinochet.
Yeah, once the word was creeping out about people being thrown out of helicopters, women raped by dogs, and American citizens being blown up on the streets of Washington DC, there'd probably be a bit more blowback.