I can believe it to some degree. Believing that what the killer did is wrong, and being unsympathetic to Brian Thompson's death isn't mutually exclusive.

They don't even need to do anything especially conspiratorial, I would expect those polls to have substantial anti-Mangione bias by construction.

Many people will want to avoid being on record as supporting a murderer, for fear of any consequences down the line. I know polls are almost certainly anonymous, but you need to trust the pollster to actually abide by that. If you have even an inch of worry, it's easier to just not answer (or answer insincerely) and move on.