If your wanting information about traitor guard more than you are wanting ideas for possible stories, it's quite simple.
Basically the Imperium of man (mostly Imperial Guard) is a vast space... and by vast im mean BIG AS ALL HELL. SO logically some of these people will be exposed to a number of different factors... whether it be the influence of Chaos directly, a lack of positive moral... say lack of help from Imperial forces, or no supplies reaching them, etc. Some of these people will decide to leave the Emperor for the promise of better lives. Some leave to join the foul ranks of the hated Chaos Marines. Others leave simpley to make their own world. Usually the foot slogging front lines men are not normally able to turn entire ranks against the grace of the Emperor. It usually falls on one of the higher ups for something so dire as to leave the world these men and women knew behind. Often times it is a commissar or psycher who is responsible. Also, if all signs of leadership have fallen; aka all commisars, psychers, lords, etc. have died... many of the troops fearing the same fate will decide to forskae the imperium and fend for themselves.
All of these are common factors as to why they would leave the ranks.
As for what life for them would be like (in the rank and files) These men, mostly depending on how they turned will be different than their zealous selves. If they fell to chaos, they will either be crazed killers, or scared weaklings drafted by the CSM themselves. If they left for self fulfillment, they may maintain some military-esk ways. But you have to remember these men are now traitors. They would be as loyal as pirates to their ship mates.
In the end, you can have a lot of fun with these guys. I wouldn't mind hearing some stories of bad ass guardsmen. Everyone knows the Space Marines are bad ass... but I'd like to see, as much as I abhor the traitor, these guys be more than just weak wimps. Especially since the day and age they live in does not allow for those to weak to fight to live for long.
Best of luck!