The original question had to be edited. A question like this is going to be a challenge because one will have to inject their own moral values into the answer. Perhaps, this is deliberate. Dubus gives us a world in which there is moral blindness and a lack of ethical vision. It might be time for us to assert that which the characters lacked.
It is difficult to compare the suffering that Richard and Matt endure. Matt losing as son and Richard losing his life is where the fundamental collision occurs. While Matt losing his son is suffering of an unthinkable level, it might be mitigated when he takes Richard's life. At that point, his suffering is muted a bit with the action he took. I cannot see him as a paragon of suffering with blood on his hands.
Richard experiences a level of suffering in having his life taken from him. While he did take Matt's son, his suffering is extended in not knowing what is going to happen to him. It is not as if Matt is happy in what he did and tells Richard, although feebly, why he did what he did. Perhaps, there is an equality in suffering within both men because they both end up living with the consequences of taking a life. Neither man is pleased with having to do what was done. Richard does not seem pleased because he killed Frank. He is not proud of what he did. In the end, Matt is not entirely pleased with what he did. Both men embraced the end of violence and are not necessarily better because of it. Herein lies a shared level of suffering.