Vengeance is righteous. Eye for an eye prevents the whole world going blind. It's only slurred as [vigilantism] and suchlike because your greatest local sinner is your black government; for stupid reasons your locals will assent to its assertions, and as such it's a no-brainer for the black government to disparage self-defence. Don't have to think about that one.
However, mortal comprehension is limited; the layman's comprehension is particularly limited. Although vengeance is righteous it is also costly. Ultimately the purpose of vengeance is to serve the avenger. Refusing to pay excess vengeance costs comes off as being merciful. Once your opponent is neutralized, at least some of the time you can leave the issue to the heavens. You can punish them by sentencing them to live with a sinner for the rest of the lives - namely, themselves. Once they are no longer capable of harming you, the issue is between them and god. Piously trust in the heavens to take care of it, even if you don't personally understand the solution.
Though if you were truly pious they would have been unable to meaningfully damage you in the first place. Inconvenience? Yeah, sure, all the time. Truly harm? Not so much.
You were only vulnerable at all due to your own sins.
Given that you were basically the one at fault - you're more or else doing it to yourself - isn't that an argument for mercy? Otherwise aren't you shooting the messenger? Nope.
As per usual, a sin in the past will often demand a future sin. A past defection will require a future deviance to return the possibility of cooperation. Sins collect interest.
In other words, once someone has sinned against you, you must respond with violence, or you will only encourage future sin. By making yourself vulnerable you enabled sin; by refusing to retaliate you only enable more sin. The righteous will not exploit your vulnerable state; at some point unrighteousness is execution-worthy. There is some level of vulnerability to temptation that should be fatal. If they have sinned badly against you, then they have revealed they are below that virtue threshold. It's time to manually manufacture some justice.
However, once temptation is deterred and the sinner themselves is neutralized either passively or actively, there is no further need. Continuing is either bullying or impiety. (N.B. bullying isn't necessarily a sin, but you probably don't want to see yourself as a bully.) That is the time where you do the-thing-which-is-seen-as-mercy.