People in-universe saying Darth Vader is cool?
I'm not talking about real people commenting on fictional characters, rather the stance of other fictional characters towards them in the same work, and how it reflects poorly on the individual espousing that view point.
But contrary to the real people, people in-universe don't know all the details.
The one time they got to see Stain outside of news reports was when he told the world that he kills to bring back the original hero spirit.
That will leave an impression on people. Villains see a man who is as sick and tired of heroes as they are, ordinary people and children see vigilante who is a huge hero fanboy.
It doesn't change that he is a mass murderer, but the fact that they got to see him, coupled with the reports that say that criminal activity actually went down in cities he was active in, changed people's cognition.
And that's why that video is always taken down and being uploaded again. The police knows that by presenting stains human side, it makes people reevaluate their opinion on him, and that's dangerous.
This is why I think that people in general (good and bad) being impressed by Stain is actually good writing. But that's only my opinion.