Emotionally disabled/ADHD boy and how to properly discipline. Any resources in the community?

I was recently hired as an after school program supervisor for 35 children. I absolutely love my job and am quickly bonding with my new students, however I'm having an exceptionally difficult time with a 5th grade boy named "James".

James is a good kid whenever he's one on one with an adult. He can have a bit of a snarky attitude on occasion without any warning or reason. When adding other kids into the mix, James becomes a nightmare of a discipline issue. He'll swear, hit, spit, and destroy anything he can. What's so severe about this is that it can happen at any time, without any provocation. One minute he's an angel, and the next he's threatening to kill other students or me.

I've spoken on an individual basis with his mother, and she has told me that he is severely emotionally disabled and also suffers from ADHD. While I've taken several courses in child psych and education, I can't follow what exactly is the problem. I'm trying to say this as nicely as possible, but it honestly seems like the only problem James has is a lack of discipline. I feel like things are only getting worse because he's been branded as emotionally unstable and therefore isn't disciplined as regularly or consistently by the role models in his life. I feel like it's not fair to discipline James any differently (less harshly) than the other kids, but others around me are telling to take it easy on him and to ignore his threats of violence. If I ignore it, what do you figure will happen when he's an adult and threatens to kill someone?

I've tried finding resources online, and now I'm reaching out to the Y!A community to see if someone can help. Thanks.