Political commissar( I believe it is what they called?) of the Soviet Union is famous of shooting their own generals and officers during ww2. How they know when a officer should be executed, is there a doctrine for them? Would them suddenly feel pity and let the ones should be executed to escape secretly? And to what extend a officer or general would do to avoid getting shot? I have seen something about it is hard to organize a army while trying to not get shot for the russian officers , why is that?

Not sure is this a correct way of asking or not, please tell me if my question is too simple to be asked here or it is a bad question.