If you consider the prequels canon, Vader was a school shooter too, and a much bigger school at that.

The situations are different. Sure we know Anakin was “a good friend,” but that then he was “seduced by the dark side of the Force.” On the other hand, we are told that Ben seemingly struggled with a darkness inside him from a young age (due to the “Vader in him”), and was manipulated and seduced by Snoke specifically. So while Vader is implied to have turned because of a greedy quest for power, Ben is implied to have turned because he was exploited and possibly (in a way) brainwashed into doing so.

I’d honestly argue that Ben’s story is more tragic, and to see him die a full villain would just be plain sad.