there is a distinct difference between leftists / socialists / communists and the authoritarian sycophants and war crime apologists that constitute the average tankie.

Qualitative and quantitative differences exist between things that still equally don't pass muster. Stealing a million dollars or 3 million dollars worth of car are qualitatively and quantitatively different, but they're both well past the threshold of acceptable behaviour.

Communists, specifically, absolutely are in the same bag as nazis and Islamists. Communism is a revolutionary ideology that requires a violent upheval and subversion of the status quo in order to establish an incompatible totalitarian political system.

Socialists, arguably, are just carrying water for communists, since i have yet to witness anyone advocating for socialism in itself and not as a stepping stone to "a classless, moneyless (possibly governmentless if you're freaky like that) society" which is just a descriptive periphrasis for communism (in theory).

But even if they weren't, I would still not agree to consider them harmless or not extreme, as their system also demands that I give up my rights or be removed.

Furthermore, what's a "leftist" to you?

The former are economic and societal beliefs with plenty of merit

I vehemently disagree with the idea that anything based on the labour theory of value and Marxist materialist analysis has merit.

They are social and economic beliefs that exist and people are allowed to have them, in a liberal society, which is more than I can say about liberalism in a Marxist society.

That doesn't mean we can't look at them and correctly identify them as dogmatic, liberticidal, and totalitarian ideological movements.