Article 119 of the Uniform Code of Military Justice deals with the crimes of voluntary and involuntary manslaughter.
The hat that Spenser has been wearing all season depicts the pirate flag of Edward Teach, better known as Blackbeard.
Sonny comments that if he were Ray and saw that the target of their next mission was the man that tortured him he would "go full Unforgiven Eastwood the second I saw his photo". He is referring to Unforgiven (1992), directed by and starring Clint Eastwood, in it Eastwood plays an aging, retired gunslinger who swore off his life of violence years ago but accepts one last job when his old partner in crime pays a visit after learning the identity of the man who murdered his wife and sets out on a quest for vengeance.