Who said morality naturally sits with the political Left?

A substack correspondent says of events since 2020:

“I was one of many who made the mistake of automatically conferring moral authority and compassion to those who espoused Leftist ideology, but I’ve since realized the danger of that. People who claim to be Leftist activists can be every bit as corrupted / corruptible, regressive, delusional, authoritarian, insufferably self-righteous, spiteful, devious, depraved, driven by envy, and narrow-minded as the ‘Right.’”

Well I’d agree that:

• Everyone is fallible and flawed, and potentially corruptible. That’s why we have institutions to limit dictatorship.
• There is always a non-trivial minority of awful people, or people who become awful through political extremism of either wing (hence Prevent in UK).
• These people will take Left- or Right-wing views. Neither wing has a monopoly on moral purity in their ranks (hence see first point above).

The fact that many made this mistake in the first place in and around 2020, is a cause for concern about how critical thinking (for clarity of definition: as opposed to critical theory) had already been eroded in the education system. Rational/emotional, liberal, critical education had been hijacked a generation earlier in favour of assumptions and teaching about a left-wing morality which struggles to live up to its ideals. 

Why would authoritarian leftism be more moral than liberal centre-rightism? Nothing actually supports that. Stalin and Mao were left-wing, but not moral. Just authoritarian, and genocidal in their fanaticism.