By that logic no one is “a leftist“. You can always just state that someone is "complicated" and therefore doesn't fit your ever-narrowing definition of "leftist".
— "Marx? he held ideas that are of the left, but he is a complicated person, he's not a 'leftist'".
— "Obama? he held ideas that are of the left, but he is a complicated person, he's not a 'leftist'".
— "Luigi Mangione? he held ideas that are of the left, but he is a complicated person, he's not a 'leftist'".
— "Angela Davis? he held ideas that are of the left, but he is a complicated person, he's not a 'leftist'".
This just makes discourse impossible. Tyler Robinson assassinated the leader of a right-wing movement and made multiple explicit statements showing a radical-leftist motivation to that assassination. Calling right-wing people "fascists" and imagining oneself as an "antifa" fighting the "fascists" is a case of that leftist motivation. Right-wingers do not call people "fascists" as a motivation to attack them. Texting "I had enough of his hatred. Some hate can’t be negotiated out" is a case of that leftist motivation. Right-wingers do not say they "had enough of their hatred" as a motivation to attack them. This is not something complicated. It is easy to understand. And refusing to meaningfully use the word "leftist" is the same as simply refusing to engage in the discussion at all.