What's alarming to me is that a lot of the commenters don't believe OP's behavior is disproportionate. I mean, his story might be true or it might be a karma farm (though it's more likely that it's true), the commenters most likely mean what they say.

Makes me think a lot more people (than I initially believed) are just one cheating incident away from snapping and murdering their SO before taking themselves out. Not that I condone cheating, I think it's one of the vilest things one can do to a partner and it's way too common (and even normal in some circles) for how horrible it is but let's not pretend getting this worked up over a failed relationship is how a mentally healthy human reacts.

OP needs serious therapy for some underlying issues that were merely triggered by this incident and based on this glimpse into his mind, I wouldn't be surprised if he was diagnosed with borderline personality disorder or something similar.