A legal process confers nothing Good inherently. It is, occasionally, a useful method to proceduralize Just resolutions to an altercation.
Tolerating Evil and letting it persist is denying Justice to its victims. Seeking and destroying it is the opposite.
I agree he oversells his own evil due to his dreams, that's an intentional joke of the manga where Absolute goes from hyper-aware to blind-spot naive with comedic timing, then insists on being "Evil" while the author shows him saving the innocent while removing thieves and corrupt nobles who have violated their responsibilities and loyalty to their people.
A lack of a legal process does make the killing quite arbitrary though.
As we were specifically talking about killing, "destroying" becomes too vague a term. I think most people would agree that destroying injustice and evil are good things. What destroying means though, varies greatly from person to person, and it doesn't necessarily follow that you are a good person for destroying evil.
A criminal organisation presenting evidence to the police to destroy a rival could be argued to be a good thing.
A criminal organisation using violence to destroy a rival would probably be a bad thing.
Neither would make the criminal organisation a good organisation though, and both probably share several problems, as either could lead to escalation of violence, harm to bystanders, and end with either organisation strengthened.
It is of course assumed that both the criminal organisations are bad in this example, and not outlawed for any controversial reason. Let's say that they, for fun, kidnap and drug people who they sell into slavery, and that all members joined out of their free will because they enjoy it.
This is even more pronounced when it comes to killing, which was the point. If you arbitrarily kill evildoers, when other remedies are available within reason, especially if it benefits you in some way, this is not necessarily a good act. It seems like the type of thinking that would risk creating vendettas.
I would also like to add that legal processes tend to also establish guilt, so that innocent aren't punished as that isn't just, which I think we all can agree on. I'm not saying though that all legal systems everywhere are perfectly functioning machines of flawless justice, in fact probably none are, but pretty much removing them entirely, or having people set up their own, is not very likely to increase justice.