Long story short, my married friend and I have known each other for 5 years and and have had a rather flirtatious relationship in those years. I have always found him attractive and have found that my lustful desires intensified over the last couple of years but would never do anything due to the fact of his marriage. I'm 95% sure he feels the same way as well due to the way he acts back. Lately he's been really argumentative with me and I feel as though maybe he is resentful of the fact that he has these feelings but doesn't want to act upon them. I still catch his wandering gaze most of the time but he seems more aggressively nasty toward me. Is this normal behaviour when lust is not acted upon?

I have many male friends who fancy me but they're never nasty to me. They can be in bad moods if home-life is in trouble. Don't forget that's still his priority.
You need to ask him what's going on. Back away and let him deal with it.
Be respectful of the responsible choice he is trying to make. You can even have a frank discussion with him explaining that you will respect his wishes and marriage IF (and only IF) you are feeling strongly in control of your emotions and think he would respond in the appropriate manner. If you think putting that out in the open might cause him to act on some of these desires don't even go there.