In the face of the tragedy he encountered and the helplessness of the injustice of the judicial system, he had no choice but to commit murder.

this is not doing justice

motives

No different from serial killers

But the difference is that neither of them are inherently guilt-free bad guys.

but the tragedy they experienced

Despair with the injustice of the justice system

That's why they chose to become such monsters

They are not bad people themselves, but in the face of the tragedy they encountered and the helplessness of the injustice of the judicial system, they had no choice but to commit murder.