To start, this piece goes above and beyond what is generally accepted as good fanart on this site. Particularly, I like the position of the standing brother and the direction of his face (to the sky, away from his gun), suggesting that he is turning away from his actions and up towards a higher power for whom he works. I also like the gradient style used for both of them with the diagonal lines. Also the contrast between the Christ-like depiction of the "Boondock Saints" and the act of brutality you notice as your eye follow the image downward really hits you, and in my eye expresses what I refer to as the Dual Aspects of God: The wrathful God of retribution and the compassionate God of forgiveness (The Old Testament/New Testament portrayals).

Two things I would change are as follows:

1. The size of the hands and guns. I feel like both of these for both men are too small. Even if enlarging them would be inaccurate proportionally, I feel increasing their size at least slightly will improve the composition. Right now, it feels like the runs are almost an afterthought and not important, and if the guns are going to get bigger than the hands need to as well, otherwise it would just look silly. Also, where the kneeling brother's hand connects to his wrist/sleeve looks a bit off, but that's only minor.

2. It may be too late to change, but that same brother's position seems to throw off the balance a bit. The composition is mostly centered and symmetrical, which doesn't always work, but it in this case if you're going to do it you should go all the way. With this in mind, I feel that he should be a bit further to the left, so that he's kneeling right behind the one standing. And if this is done, maybe the corpse should be moved to the left as well as cropped off by that much, just so he'll have some space and not be kneeling right over the guy.