It's a combination of their religion and the unrestricted practice of it. But it's not unique to Islam itself or Muslims.
Christianity preaches the same shit (literally, it's the same God even - the God of Abraham). In the Bible you have orders for genocide, murder, killing babies, killing of homosexuals, violence against non-believers, killing adulterers - with you ending up on God's shit list with a ticket to hell if you fail to do any/all of it. Like Islam the Bible also condones incest, pedophilia, rape and slavery. Jesus reinforces it in the New Testament for all believers as well, much to the chagrin of folks who say "but that was the old covenant and only for the Jews!". Parts of the world that follow the Bible but have a similar lack of societal standards behave in the very same way and treat their fellow humans identically. The reason you don't see it in the west with Christianity or even people of other faiths that have adapted is because they know deep inside they don't really believe it, would never do what the deity commands them to do and therefore aren't nearly the threat these people are. In the west, a person that behaves like this would be called an extremist or "not a real _____" when in reality, they'd be the only ones who were truly following their holy texts.
You get similar behavior in tribal people or even atheistic sorts who also haven't had any kind of cultural evolution. You see it far more however with religions (particularly the Abrahamic faiths) in these places because not only do you not have any source of evolved morality, you have an all powerful being in your imagination and on paper literally ordering you to treat people like this and threatening you with eternal suffering if you fail to behave like an animal.
This isn't just your brain on Islam. This is your brain on a number of different faiths (especially the "big 3") without restraint. No one to tell you "no" and a big bad bully in the sky telling you "Do it. Do it.". You just don't see it here because people had enough foresight to ignore the fuck out of the Bible and other holy texts and made laws that directly conflict with God's orders, as well as proper education resulting in a population that by and large knows better even if they go to church on Sundays pretending they don't.
In the Bible and the torah it says to do the same thing as Islam is really true everything you say?