This is Butcher's background story, part of which is background of real war but, as he usually seems to do, Ennis manages it with no glorification, rather a great deal of respect for the people who have actually done it. Butcher's grandfather's single sentence is an outstanding moment.
For me Butcher has become a really well thought out character, not just a recycling of wafer thin superhero/villain clichés of righteous vengeance. It turns out it isn't just a case of him being capable & justified in his coldblooded vengeance simply because his targets did something similar, he actually seems capable of doing it because he's pretty close to a psychopath. The genetic bit of his psychopathy is from his father but it requires an environmental trigger to tip him & his brother, mother & wife gave him the environment he needed to control it. But without that environment then his genetic traits are beyond his control, so is he really responsible for his own (unjustifiably nasty) actions? It just poses some really interesting questions.
I honestly think Garth Ennis has made a very important contribution to the comic book canon with The Boys, but I would recommend stating at the beginning of the series rather than jumping in here.