Although the jury ultimately finds that Wallace was temporarily insane, the testimony and the expert testimony on this issue were pretty one-sided towards the other direction. Wallace admits that he puts the gun in his briefcase with the purpose of shooting a stranger on the street - a somewhat criminal intent. Wallace was meeting with his employer to persuade him to give Wallace his job back. That's not exactly a scenario for suicide. Nevertheless, Wallace says that he decides to kill himself after his employer refuses to rehire him.
Life is just too hard.
So Wallace kills him. While this string of events may be unlikely, it is not irrational and the verdict simply did not seem credible. This is what is known as an assault. Neel goes a tad overboard and cuts off Hinks's head. Unfortunately, Hinks will no longer be guest starring in future episodes.