That is practically a textbook example of a slippery slope fallacy.

FTFY: you missed the most important word in that sentence. You don't seem to comprehend that "slippery slope" is a fallacy.

The general public is capable of evaluating circumstances on a case-by-case basis. That does not change just because Mangione is acquitted. We are perfectly capable of recognizing this case is a rare exception, and not a general rule. Previous rare exceptions have not resulted in rampant vigilantism; there is no reason to believe that this case would be different.

The aphorism you should be considering is "following a line of reasoning straight off a cliff". We don't have to do that. The first three words of the Constitution tell us we don't actually have to apply a law if we really don't want to.