So we know that the Orlando shooter was a radical Muslim who swore loyalty not only to ISIS before the attack but several other Islamist organizations; it is rather confusing considering that many of these organizations that he pledged his allegiance to actually hate each other. According to an article in the Washington Post, Mateen Omar may not have understood the differences between the groups.
We also know that he might have been gay. People from his past admitted that the perpetrator asked them out on a romantic date. Looking further into him, investigations found that he actually used gay dating applications. He also frequented the gay bar which he shot but this may or may not have been due to looking for a date or a drink but plotting his attack. It is difficult to say.
http://www.palmbeachpost.com/news/news/orlando-shooter-omar-mateen-was-gay-former-classma/nrfwW/
According to his father, he was set off by watching two gay people kiss. Now, from all of this I can discern a potential hypothesis as to why he conducted his attack. He was a self-loathing and insecure gay person. The terrorist, who was reported to be quiet and awkward, may have been in conflict with his religion and sexuality. I cannot however fathom what manner of process it takes to cause such a perversion that one would seek to commit the atrocity that he did. It may be that by killing the homosexuals at the bar that he was somehow demonstrating that he is unlike them. Or, perhaps it was his way of rescinding himself spiritually.
Considering that many people probably have this conflict of religion and sexuality, most don’t do what this person did. Many Christians may have this internal conflict and live in misery. We don’t seem to see a streak of terrorism coming from these people however. The issue of turning this conflict into terror may reside in some tendency for certain Islamic views to become more violent. Although violence can be extracted from the bible just as the Koran, the interpretations that lead to religious terror is in our times more prominently seen being applied to Islam. So it is not the text itself but the popular interpretations floating around through various platforms that someone like an ISIS recruiter or hate preacher could use like TOR or twitter or something. The world at this point knows very well that the Koranic interpretations of violence are in great abundance these days. On the other hand, we see less of that from other religions.
So I would say that radical Islam is the culprit. It caused his conflict with being gay. The final solution to his problem was obvious to him from the lens of radical Islam. Take an assault weapon, go to a gay bar in Florida, and kill as many people there as you can before you die.