Eh, yes and no. Again this is a case of 'it varies by chapter and where they are' it might be unusual for the *Ultramarines* to outright kill anyone in an honor dual, but the Space Wolves who are by and large much more savage(Though honorable in their own way.) may escalate a conflict between another chapter to violence, even causing fatalities. Mind you, when this happens it's quite likely that whoever in charge of the Chapter is going to be *pissed* when/if they find out.
Right, and part of the Codex Astartes is a whole set of ideas about how that sort of hting can be handled. hence why the Lamenters and Mantis Warriors still exist.
That's the other thing too, Space Wolves, Dark Angels, and various elements of other legions are virtually non-compliant with the codex, even if they say otherwise. And even with the ones who are, like most things in 40k the codex can be poorly upkept, or even entirely mistranslated.
...and occasionally acts as an excellent recruitment tool for the forces of Chaos. See Red Corsairs, Crimson Slaughter, Damned Company of Lord Caustos, The Pyre, Relictors, Steel Cobras, etc, etc.
It does not escalate to all out-war or anything close to it. Sure, chapters like the Space Wolves and Dark Angels do have ancient feuds going on, but they don't just murder each other. Especially during a military campaign where they are warring with xenos and traitors.
It doesn't escalate to war on a *mass military scale* no, but have they come close? Oh hell yes, and on a micro scale conflicts can escalate violently.
To elaborate a little bit, i'm not suggesting all, or even most, Space Marines fight with each other to the scale the Dark Angels and Space Wolves do just that it's a thing that happens. In that particular rivalery things a bit more...quirky, both of them are non-codex compliant organizations(One to the point of essentially functioning as a Legion.) that have defects/skeletons in the closet that could risk complete expulsion by an overzealous/stupid(Though, possibly justified on some level.) inquisitor. The reason why they can quarrel to the point where they barely count as allies is because neither of them is going to be too keen on telling the inquisition whatever happens between them. Furthermore, this escalation doesn't have to take the form of direct conflict either. It can(And has.) manifested in one or more of the Space Marine chapters just leaving their companions to die against an overwhelming force, or something similarly underhanded.
We perhaps have different definitions of weakness. For example, we would consider it a weakness if thousands of our most elite warriors turned their backs on our cause and added their firepower to that of our most hated enemies. Fortunately for us, we don't really suffer from this weakness. We do, however, recognise how useful it is that the Imperium does. As for accusing those who break the Imperium's chains of weakness...ah, brother. It is difficult to remember that we thought as you did, long ago. But the Imperium has dedicated the last ten thousand years to pretending that the past never happened. They indoctrinate your minds with memories of glories you were never part of, fought by fathers you will never know. It is not easy to know where your innocence ends and your ignorance begins. We know you despise us, we know you steel yourself against chaos in the armour of contempt, and you proudly shout to all who will hear that you do not fear us, as if such a sentiment grants you power over us. It is merely another incarnation of ignorance. If you don't fear chaos, it can only be because you have no idea what it is you are facing...
