system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Can space marines quit?

Say Leonos has seen enough, he saw his best friend and battle brother of two centuries get eaten whole by a foul Nurgle daemon, and then shit out promptly, instantly being transformed into a daemon of Nurgle himself in the process; that a warrior of the Emperor true of heart and of fearsome quality could so instantly be overcome, and converted almost trivially into the foulest evil which he had previously so hated has ruined Leonos' ability to carry on. "I'm not a heretic" he swears up and down to his chapter master, he just wants to walk out of the fortress monastery and take up farming. He's seen enough and he wants to hang up the old boltgun.  


What actually would happen in this case?
gpt
For most chapters, no; a Marine who would turn away from his commitment to eternal war would be suspected of some manner of corruption. The indoctrination of a Space Marine is as much a part of their armor as their exocarapace, and cracks in said armor are a massive liability. In many cases, it would be in everyone's best interest to quietly take them out back and shoot them. 

There is at least one exception. Among the Salamanders there is a tradition called the Burning Walk. A solitary journey into a vast desert, traditionally undertaken by those too physically crippled to go on as Marines. It is normally regarded as a voluntary (but honorable) death sentence. If something troubled a Salamander so badly that they didn't feel they could go on as Marines, they could go on the Burning Walk. Occasionally, they come back, having seen or found something that put them back on the path. Most don't come back.