"You failed! Die!"
"You saw what happened to your superior, and now you've failed as well! Die!"
"You lived and your superior died in that battle. Die!"
I'm actually curious why there aren't more defectors or more Sith shot in the back.
I think, small amount of defetions in Empire comes from mentality of Imperial peaople - like in many totalitarian systems, they kinda "brainwashed".
Second thing - killing own is quite common in Empire - you name only small amount of reason of death in Imperial Armoed Forces. Guy named Darth Acharon was killing his soldiers almost for anything - from mission failing to bad boot polish. Numbers goes in hundreds.
Thing is - Imperials has less men than Republic and still wasting them in stupid causes like Siths personal desires. But hey, they got money from slavery so can hire Mandos and other mercs