Tl;dr: an NPC that had been tagging along with the party got the final kill on a big bad that had traumatized her.

A bi-weekly meetup group on Discord finally settled a quest that had been pursued for several months. A little girl named Chisha was being subjugated by a doppelganger. It took on the appearance of her father, who was keeping her alive because were he to allow her to leave, there was a possibility that his secret would be found. So the group rescued her, traveled to a nearby cave where her father had been found, and expressed their desire to put this evil doer to rest.

Following a battle that took the entire 3-hour session, with minions and heavy spell casters,, she summoned a balgura and it used its heavy fists to pound this guy into the dirt. This was after several attempts to get the battle to pause, or at least slow down with suggestions and commands to get him to stop, which did allow the PCS Lop off his head, if they so chose. This did not happen, and she got the final blow after a severely buffed HP pool. I feel like The Narrative of her getting retribution fit better than another Kill from a player, but after the battle was done the Warlock did in fact the verbalize that he was a little saddened that he didn't get the kill, it was his idea to go back and pursue this doppelganger to end his streak of evil.

I know in the grand scheme of things, this is probably not that big of a deal, but I strive to create both a cohesive story, and to satisfy my players. I know things don't always go the way that they're supposed to, but a part of me feels like it was dissatisfying for the PC to go off on this venture to correct an Injustice and it was taken from him. I don't suspect that anything like this will be happening again, but more or less, AITA?

update his response- "Oh, no worries. Rollo was unconscious at that point and if anyone had a genuine grievance with the shape changer, it was Chisha."