This image from the Rogue Trader rulebook has always intrigued me. It suggests that not all space marines were law abiding loyalists locked away in a fortress monastery waiting on the call to blow away some xenos. In fact this pair look like a couple of guns for hire, waiting in a sleazy bar for their next job to come along. We all know the way the emperors finest are portrayed these days. Noble superhumans with indefinite life spans, educated and intelligent, fearlessly defending humanity without a second thought. It wasn't that way in the days of Rogue Trader.
In the early days the marines were depicted as being recruited from hive worlds, prison worlds, feral worlds - any where a large number of psychotic killers could be found. These dangerous scum would be rounded up and indoctrinated by whichever chapter needed them. Implanted with all sorts of bio-mechanics and using drugs and industrial scale hypnotherapy turned into controllable killers. Squeezed into suits of powered armour they were sent off to the most dangerous parts of the galaxy to fight on humanities behalf, far away from the people they were defending. No wonder they needed Field police to keep an eye on them.
Considering their background it wouldn't be a surprise if the occasional marine slipped off during a mission and set off to see the galaxy with his new found strengths and skills.
This guy could be an escaped marine living his own life in the galaxies arse-end.
Inter chapter conflicts would be a lot more easily explained if we take this point of view. You can imagine the Badab war being fought as a series of grudge matches between hot headed super psychopaths egged on by their officers to uphold the honour of their adopted chapter.
It also explains the need for chaplains in the ranks, to ensure that all the hypnotised, drugged up, genetically enhanced, power armoured psychotic killers are doing the right kind of killing for the right kind reason.
This kind of merciless use of humanities most dangerous as a bulwark against the dangers of the cosmos obviously has an effect on life expectancy. A marine that had survived 10 years would be deemed a veteran with metal studs fixed to his skull.
I like this view of space marines. I want them to be barely controlled killing machines. I want them to be renegades without having to worship chaos. They never worshipped the emperor, why would they suddenly get religion upon becoming deserters? So from now on that's how I'll see my astartes, mentalists with big nasty guns barely managing to keep their weapons trained on the enemy. A danger to humanity as well as xenos but deemed to be a necessary evil for the imperium a sake. A army of supermen that still need to be policed, just in case......