That's a really interesting comment.
I think that for many, the fact that Charlie Kirk had pretty unpleasant views isn't necessarily the issue, so even if it was a lovely person that was murdered, I'm not sure it would make much difference. Sure, you might feel absolutely terrible and it would weigh heavily on your conscience, but turning your child in, knowing they face the death penalty would be worse.
I think killing a child might make a difference though for many. I think fewer people would be willing to stay silent.
Maybe the other thing is that not everyone has the same relationship with their adult children. We see plenty of posts on here where people seem to have quite chilly or distant relationships with their adult children - and we don't know whether that's colouring some of the replies. If you're not that close to your child, it might feel easier to turn them in. Not easy, but easier.
The other thing that occurs to me is that we're all imagining our children as we know and love them; presumably we all think our children are lovely people. I know I do. Maybe if your child was violent, abusive, and aggressive, and you'd watched them abuse a partner in the ways that you describe, people wouldn't be feeling the same way.
And yes, you're right. The death penalty is barbaric and frequently inflicts enormous suffering. I would struggle to agree with that for anyone, it's inhumane. Absolutely zero chance I'd send my child to face it.