Vindictiveness

Etymology

From Latin "vindicta" (vengeance, revenge), from "vindicare" (to claim, avenge, set free). Strikingly, "vindicate" — to clear of blame — grows from the same root; the difference is whether one claims justice or exacts revenge.

noun · /vin-DIK-tiv-nis/
A disposition to seek revenge and nurse the desire to punish.
What it names
The trait of not letting a wrong go — the appetite for payback held long after the injury.
Example
His vindictiveness outlasted the divorce by a decade, surfacing in every custody hearing.
Related idea
Resentment broods on a wrong; vindictiveness acts to repay it. One is a lingering feeling, the other a settled will to strike back.