And this is why I say I don't have an opinion on his killing.

It's really easy to lie online (both mechanically by just taking stuff out of context as you showed and because people face no punishment for it).

That being said, if I had an opinion, only one of the possible sides is OK to say publicly and I think that's wrong.

For example, one of the claims I heard is that he was orchestrating/supporting harassment of college professors through his Turning Point organization. Now, harassment can lead to suicide, driving innocent people to suicide is murder, a just punishment for murder is death, and when a punishment is just then it doesn't matter who carries it out as long as they have a sufficient standard of proof.

So depending on whether that claim is true, how "successful" he was and the morality of several logical inferences (which is subjective but for each statement/inference you will find a lot of people supporting it), a sane and rational person could perfectly justify the killing.

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Most people are hypocrites. And what they really hate is when somebody tries to apply a consistent moral system to themselves and others. Not just because it puts them to shame (most people never try to be consistent and they know it even if they'd never admit it out loud or even to themselves). But also because when you apply consistent rules to severe offenses, you very quickly get very severe punishments and people are not comfortable with the idea that they too could be punished in this manner.

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As another example, I hate when people try to vilify him by quoting him saying some dead children are OK because that's the price for the right to bear arms. Yes, innocent people (including children) dying is sad and wrong. But what do they think happens when nobody has guns? First, people would still kill each other using other tools, even mass murder wouldn't magically stop because car ramming attacks seem to be on the rise. But, second, a long term effect is that people in positions of power who are unaccountable through legal means are no longer accountable through extra-legal means.

One of the first things every dictatorship does is restricting access to guns and confiscating them.

Yes, high gun ownership has a continual price but low/zero ownership has a much higher one-time price and after that it no longer matters what people think.