Is there a command to force kill move to always be on for Summoned creatures?

There is such a mod to do this. The Dance of Death - A Killmove Mod

This mod has everything to do with kill moves and the such, and setting that up would be easy.

Just use the Dance of Death power, then set the kill move chance to 100% and that should do it. Now your companions should always execute a kill move on an enemy whenever the enemy's health is low enough. Also, you can set decapitation chance so if you like seeing heads flying, this sets it to always do that as well.