There is an option to turn off elaborate execution of bosses. However, I think there should be an option to turn off executions of bosses completey, if you turn it off, the bosses just die normally (like the Leman Russ tank which has no execution animation and just explodes).

The reasoning behind this: when you start to perform the execution animation, the player becomes invincible (which is not a problem, as otherwise you can die while you have no control over your character), and the mobs loose the aggro on the player because of this. It is really annoying with melee/close quarters characters, as the mobs you grouped up, startg to disperse and return to their original position, and you have to group them up again.