What determines if an opponent is "beaten"?

[Aya]

In some sessions, I've seen the Grim Reaper try and slap me with his arm stumps, while other opponents will sometimes cower and whimper over the lightest bump on the head

[Someguyinhere]

They're dead or the AI has given up.

[Giga_Bigga42069]

when they stop moving and making noise

[Jakkar]

Morale seems quite variable from individual to individual, and the damage model is nuanced enough that you can't always see how much damage/pain you've caused.