Vocenoctum
First Post
This.
It adds verisimilitude if sentient, free-willed bad guys try to bug out when it becomes clear to them that they're going to lose.
I had a couple encounters when I DM'd where the villain's position had obviously become untenable and he fled. (In one case, the guy actually still died from a lingering effect, but he sunk to the bottom of the lake and the players thought he'd escaped...)
The players hated it, most expressing the idea that they had "failed" the encounter because there were no corpses at their feet. (This aside from any loot issues.)