James suffered a traumatic injury after falling off of thea balcony of their home during a fight with "some guy" who'd posted intimate pictures of Bella online without her permission.
Bella: I sent these pictures to some guy. And then when I woke up the next morning, they were all over the internet. They were everywhere, everyone had seen them. Everyone at school and my Mom and my brother...
Maxine: So, what happened?
Bella: James wanted to find the guy who did it, and he lived in a three-storey apartment building. There was a really big fight and .... he [James] went over the balcony.
The ending suggests that he wasn't just comatose, he was in a persistently vegetative state.
VO: But two years prior to the murders, James was in a coma, a vegetative state.