I have felt strange vibes from the beginning about GM. I watched his first testimony on video stream, no idea where it was going, noted that the timing of when they were at lunch must be important, and that his having to call NC back and tell her (now everyone) that he had failed the exam. I attributed vibes to his having his exam results made public.
After rewatching this and his other testimony, I believe GM was irritated at BC, resented BC's achievements and upset by his recent lack thereof, and saw behavior by BC as deliberate slights to belittle GM. Remember how he described BC answering NC call and going outside. I think this was building over a long time, becoming intense after the murder.
Being the hero, tinged with a bit of payback.
IMO Another person wanting to help protect the prosecution's case, frustrated when challenged by defense.