military commanders decimating their own units for failures was common.
It's funny you should use that word, because the word itself originates in the Roman practice of killing a tenth of your troops to punish them for cowardice or failure.
Anyway, since they're the token bad guys obviously the Sith come across as comically cruel idiots. Lucas said he modeled the Empire after Nazi Germany. What people fail to take into account however is that pre-war Germany was not a bad place to live in if you were just a regular German and did not belong to one of the persecuted groups. This is why the Empire comes across as comically evil, because they treat everyone like crap, including their apparently so beloved Humans. Maybe this is being done so the bad guy always loses in the end... but the real Empire could not have worked for so long on a basis of random cruelty and violence. You have to give people a purpose, something they all unite for. And "if you don't get killed, you can maybe get a bit of power" doesn't cut it.
There are so many times where you just cannot believe how little sense a set of choices made. Where the Light side option is the only way out because the Dark choice is just too illogical or plain stupid.
And I found the SI story line to be insanely boring to be honest.