how would we know some deviant hadn't just convinced them to do that?

Ok let's see the 4 protagonist of your scenario :

The Child, declaring full self ownership and responsibility.

He is responsible for his deeds. He claimed it.
If he has to suffer consequences on this, so be it.
If he survived he may teach other, if not he will be an example of what not to do.

The parents of the child.

They are responsible to teach the child the consequence of his choice.
Yes it's hard, yes being a parent, aka responsible for another being, is a hard task.
It is overlooked our days and should not be…
If the evil one convinced you child leading to him being harm, you failed as a parent.

The people living around this drama.

We are dealing with a huge problem here, in a wise society, these kind of behavior, convincing other to do harm or harming themselves should be punished heavily.
Put it bluntly, if a creepy pedo convinced a children of the neighborhood to declare himself being adult for him to abuse the child, you have to remove this threat (kill) him.

The evil one, convincing the child.

This kind of twisted mind I find no excuse.
Because, ok, it started with a (strange for me) desire.
But then it's followed by scheming, deceiving, lie to end by abusing a children.
He caused massive harm, no excuse death sentence.
By the children, the parents or anyone witnessing the thing.

As you see, in a wise and courageous enough world, this scenario would not be such an issue.