From my not incredibly extended experience, the problem was the FFL attracts exactly the sort of people in op's situation - those with criminal records and/or nowhere else to turn. You're then basically a mercenary in a group of other very fucked up mercenaries, and from the outside it appeared a great way of reinforcing the worst aspects of one's personality.

By contrast if you simply go live in South America and teach English, you'll just be any other expat, and immersed in a normal local culture much more likely to help you become a sane, well-rounded human.

(for reference, I spent two weeks in a hostel in Marseilles with a creepy Algerian criminal in the FFL who'd been wounded/furloughed and was trying to obtain French citizenship along with a 19 year old in-denial gay Mormon who was attempting to join. The Algerian wound up making violent/rapey comments to at least three female friends)

You might enjoy: https://www.quora.com/What-is-the-hardest-thing-you-have-eve...

Also worth reading Legionnaire by Simon Murray, although he seems to be more positive about the long term impact on his life.