I think the defense of Stalin comes at the end of a particular path that can be very appealing to people for various reasons.

One potential driver of it is that ML/Stalinist groups are not too dissimilar from a secular religion; it has a group of people ready to welcome you as a friend and ally as long as you agree to a certain worldview and a very specific reading of history from approved texts that always pose historical Maxrist-Leninists as righteous figures who didn't really do anything that bad, and if they did, it was for the greater good, and justified.

Those texts can even make a certain amount of sense if you're disillusioned with the status quo, and distrust western media. It's also likely extremely comforting to believe that while the western world is fucked up and exploitative, there are at the same time powerful allies elsewhere in the form of the AES states, which in their view are making steady progress towards the promised socialist utopia.

So ML groups can offer a feeling of belonging, friendship, a comforting worldview, and the belief that if we just follow the directions of long dead prophet-like historical figures (like Lenin or Stalin), then we will someday have heaven on earth. These are extremely appealing aspects to someone who may be very lonely, or who may have suffered a severe trauma and may not have their basic needs met (which may also be what leads to some people being attracted to the MAGA cult)

To someone well versed in history and a desire to find multiple viewpoints for a historical event to avoid propaganda bubbles, the true nature of ML/Stalinism and its authoritarianism becomes self-evident. But for those who never went down that path and are in a vulnerable state, a 'scientific' cult offering you hope, meaning, and companionship is very easy to fall into, and thus eventually willingly self-delude themselves to attain in-group status.

Just like with normal religions/cults, once they are deep inside it, they are heavily encouraged by the in-group to suspect any outside information that challenges their narratives or isn't approved by the group, and thus the cognitive dissonance they could create if looked at more objectively can mostly be avoided.

Also similar to religions; a ML member is strongly encouraged not to have doubts about the validity of the approved sources/texts/history. If doubts are voiced, the group will attempt to re-affirm the validity of the texts (keep the faith). But if that fails and the member continues to voice doubts, they are likely to be ejected from the group, which is very traumatic for most people, but especially so if there is no other support groups to lean on. This likely results in many keeping doubts to themselves, or convincing themselves those doubts are just CIA lies, similar to how Christians try to reject their own doubts with the concept of Satan spreading lies to tempt a Christian from their faith through logic or archeology.