"When a young Imperial Agent succumbs to the Ruinous Powers, it's a crime.
When a Rogue Trader of the Emperor does, it's a loss.
Yet when one of our own falls, that's a tragedy."
--- High Chaplain Marqol Tomasi of the Crimson Fists Chapter
Well, I think the Space Marines in Deathwatch are heroes just too perfect. Great heroes must always be surrounded by a bit of personal tragedy. To that end I have devised Temptation rules, mechanics for modeling a latent weakness in a Space Marine. A small tiny vulnerable spot which may (but for most it never will) devolve into a gateway for the lure of the Ruinous Powers - who will readily and happily try to exploit it... for the Emperor's grand-sons always get a special amount of their attention, trying to lure them to darkness. This way it's possible to model the fall to Chaos in a unique way.
The system works like this: there are categories of "weaknesses" loosely based on the christian Seven Deadly Sins. With using this mechanic, a Marine's hypno-indoctrination is rarely perfect (contrary to popular belief based in imperial propaganda) and as such a Marine may have a weak spot, in fact most do but they have this fully under control and as such the issue never surfaces. Yet it's through this weak spot that a Marine at the end of a long road can fall to Chaos.
The system itself is supposed to work via "Temptation Points", representing the degree by which a Marine is willing to betray the Emperor and his Battle-Brothers. Temptations Points add to Corruption Points for the sole purpose of determining whether a Marine has crossed that all important 100 point barrier and is lost forever.
Since Space Marines rarely succumb to Chaos, the system is designed on a sliding scale: initially it will be very hard to accumulate Temptation Points at all - only through a tempting daemonic force targetting that specific vice or through a very unusual, very strong and mundane, real world temptation regarding that aspect of spiritual vulnerability (aka very rare ). This allows the GM to hand-place such occasions into a scenario, allowing effective callibration of the danger of the lure of Chaos by the GM. These temptations will then act as if "spiritual landmines" - carefully placed in advance by a GM in his missions.
Yet - as soon as ("if") a certain amount is reached, the temptation will grow and the resistance weaken; normal, less extreme triggers for that temptation will start to gnaw on the Marine inside, possibly even ending with the Marine going AWOL and indulging in whatever sin afflicts itself on his mind. At this point it will probably take a heroic action of a Chaplain to save the Marine from eternal damnation. Of course a Chaplain may also detect the temptation earlier on and help the Marine ward it off.
That's the basic idea, I'll post details later... I'd like to have some feedback from you folks.
Any thoughts?
Alex