Are there quite a few people who served in Afghanistan that ended up on drugs?

I'm sure there are. There's a different mentality these guys have to be in to serve in a hot zone. That mentality is one that allows you to pull a trigger on other humans. Not everyone can block those memories off when they return to civilian life, and as their mentality returns to a killing is wrong mindset, then their actions however justified at the time really do haunt some of them.