[@Jbcool] I get what you're saying, but that wouldn't necessarily work for a couple of reasons. First of all, with the Primarch knowing that their civil war will see opposition in the form of stronger entities than themself, they'd ideally have the presence of mind to do what they could to make sure it remains under wraps for as long as possible, until it's effectively too late to stop it by just slaying the leader (and to wit, the most likely cause of such a civil war would make them not care so much about being threatened by the Zodiac Guard). Secondly, there's the slight problem that Space Marines are, as a rule, absolutely loyal to their Primarchs. Killing somebody slightly less superhuman than yourself is one thing; getting past ten thousand or so of his bodyguards to reach him is quite another. And yes, regarding [@agentmanatee]'s point: if the instigator has been Chaos-corrupted, there's a good chance he's ended up with absurd amounts of Undivided blessings, enough to easily take out an Armatus.

Incidentally, it may be the case that the Bolter and Chainsword website for generating Astartes armour may not be outputting images correctly. If anybody can provide a more functional option, that'd be excellent; otherwise, I may need everybody to describe their Legion's armour in text rather than images.