The beginning of your paper comparing the characters of Montresor and Matt Fowler shows the correct structure for your thesis, but I don't agree with the original thesis itself.
In Poe's "The Cask of Amontillado," Montresor is presented as a man with the maniacal need to murder Fortunato. The reader is never given any reason for Montresor's anger, but he is an evil, calculating man who has planned every step down to the last detail; when he walls up Fortunato, Montresor has no regrets. Many years go by and no one suspects that Montresor had a hand in Fortunato's disappearance and death. Montresor has no misgivings about his actions, feeling perfectly justified.
However, Matt Fowler is a very different man who is driven to do the unthinkable: to murder his own son's murderer. His love for his son is what drives Matt to not only contemplate, but to carry out his plan.
The second part of your thesis, what you are trying to say, I DO agree with. It could be argued that by killing Richard, Matt becomes a version of Richard as well: he becomes a killer. Matt is the devoted father and husband, and it seems impossible to imagine such a man taking this step. However, you might consider whether he would have found it easier to live his life knowing that his son's murderer would be released from jail for that murder when he was forty-six, free to live out his life as he pleased, or if Matt will be better able to deal with the knowledge that justice has been served in his eyes: an eye for an eye.
The line that seems to separate the two men is the question of Matt's morality and Montresor's madness; I do not believe that Montresor has any morality. He is probably a sociopath in that he feels no guilt and no remorse at all. Matt's morality will be his judge and jury: the real question is how well he will fair raising his other children, and living with his wife with the daily knowledge of what he has done— there in his mind when he awakes and there at last when he falls asleep. Will he ever be free enough of it to find pleasure in his life? But was that possible at all once Frank was murdered?
You have some wonderful material to work with. In summary, if I have it right:
Matt Fowler in "Killings," and Montresor in "The Cask of Amontillado," are similar in that they both are murderers. However, Matt murders because he must find justice for his son's murder, and Montresor murders most certainly from madness. The main difference between these two men is that Matt will have to find a way to live with the truth of what he has done, while Montresor will dismiss it from his mind.
Shave it down a little:
Matt Fowler and Montresor are both murderers, however, whereas Montresor will give his actions no further consideration, Matt's sense of morality will battle with his need for justice for the remainder of his life.
Hope this helps.