There is no rational explanation. The logical, moral, civilized mind cannot comprehend such evil. But, there is an explanation for it that starts back at the very beginning.
The arcangel Lucifer lusted after the power of God the Creator. In punishment God cast him into the fires of hell, but evil had already been loosed upon the world.
There are still the same two choices: right and wrong; or saying it another way, good and evil.
My personal conviction is that mooslims and the sundry followers of their miserable, hateful philosphy are most certainly not in the "good" camp.
In the end times, the Scriptures tell us the final fight will be the godly against the ungodly. As for me and mine, we stand for and with the godly.