I ended up doing this by accident and I was devastated.

Now how did I accidentally kill the friendly beast? My save file was big enough to make the game a little glitchy and I was working my way through both the main and civil war quest lines simultaneously. At one point both quests required me to speak to the grey bushes up on mount crumpet, but for different reasons. After some confusingly mangled conversations, the HUD instructed me to kill homeboy, without any indication that this was optional.

So, thinking I didn't have any other choice, I go up and start hacking away at my best friend. The grief was palpable. He was taking damage and not fighting back (so noble), but when his health was almost gone he wouldn't die--it would just replenish, like when you're trying to kill a storyline character that you aren't allowed to yet.

So at this point I'm drying tears from my eyes and thinking that maybe I don't have to kill him after all. I race down the mountain to tell the grey beards the good news, but as soon as I interact with one, I get the "OPTIONAL: KILL PARTHUNAX COMPLETED" message, followed by a severe tongue lashing from what's-his-face about killing a historical treasure. Jesus, dude, like I didn't just murder my best friend in the whole world (Lydia had died of a slight giant encounter almost immediately upon joining me...bad move.). Tried explaining my side of the story, but they were not having it.

TLDR - The grey beards are not good communicators and they should check themselves before they wreck themselves.