In S05E17 of Buffy the Vampire Slayer, Ben just stabs Jinx in the stomach after Ben accidentally reveals something. Why stab only? Why not kill?
The original script indicates that Ben felt that stabbing Jinx would be sufficient to kill him. Note also that he's described as acting with desperation (twice) and is terrified of getting caught standing over a body, so it's pretty clear that he's neither thinking straight, nor wanting to hang around to make sure he's finished the job.
JINX: Please. I heard nothing-
Ben grabs a knife from Jinx' own belt - desperate.
BEN: I can't let that happen. Don't you see?
With that, Ben brutally STABS JINX in the abdomen. Jinx cries out and then collapses in his arms. It's quick and brutal.
BEN: I can't…
Ben's eyes dart around desperately. He's still alone on the path. After a terrified beat, Ben drags Jinx' body into some bushes and dumps it. Then he takes off - leaving Jinx for dead.
Since Jinx has made a nearly full recovery just a few weeks later (returning in Buffy: Intervention with a painful stomach-wound but clearly able to stand/walk around/ass-kiss) it should be clear that what Ben felt was likely to be a deadly strike wasn't actually deadly to a demon.