Islam and Christianity are probably the only two religions among many others that actually implicitly teach their followers that they will not incur any bad karma for killing another person, as long as that person is not of the same religion.
While many, if not all, other religious followers committed murders in one way or another throughout human history, we cannot say, "That's what your religion taught you to do." It was all solely the blame on the followers, individually. But with Muslims and Christians, we can actually say exactly that: "That's what your religion taught you to do."
Sometimes I wonder why some of our hearts cannot differentiate what is good teachings, and what is bad teachings. How come some of us (or should I say, a majority of us) do not question when a religious teaching call us to murder another just because of embracing a different religion?
It is almost as if our heart inherently does not have justice, fairness, empathy and righteousness. So much so that if our religion teaches us to violate another, or even to ourselves, we will willingly follow the order without question.
"I am a jealous God."
If that is so, then aren't you an inferior one? Because your emotion precedes you.
If a God is supreme, then why is there still jealousy arising from His heart and emotion?
Why is He, being wise and all, not impartial to religious differences?
And most importantly, why are many of us blindly following a particular religion even if it imparts teachings that blatantly violate the Golden Rule?
The Golden Rule: "Do to others as you would have them do to you."
Do you want others to kill you because you embrace a different religion? If not, then why are you killing them for being different?
"Because my religion is superior."
Therein probably lies the main reason why most of us embrace the religion we are in. It's probably because that particular religious teachings resonate with our nature.
If I was a sex pervert, I would probably readily embrace a religion that purport to worship a God of Sex. Because by worshiping this God of Sex, I would get what I selfishly want, i.e. sex, without regard to how and where I would obtain it, whether it would be from raping, abusing, exploiting, etc.
Similarly, if I was a tyrannical person, or a domineering person, or a person that always delusively think that I am superior and therefore everyone else must submit to me and worship me, I might readily embrace a religion that teaches the concept of submission (to a higher authority) and dominance (on the weaker ones). This is so that I may have the opportunity to (as well as the religious justifications) to impose my double standards on others and dominate them.
This is probably why those that were born into certain religion never question its teachings of murdering others not in the same religion as they are. Otherwise, if they have a heart of humility and compassion/empathy, they would reject such religion as they learn more about it.
Ultimately, why can a religion dominates a certain segment of the population? It is like I said, probably because the teachings of this religion resonates with the nature of this population.
This is why we see different people, with different nature, embracing different religion teaching different things. Birds of the same feather flock together.
If only the human heart is pre-dominantly humble and compassionate/empathetic, there would be heaven on earth since time immemorial.
A religion that teaches war and murder for whatever reason, is a very inferior and primitive religion. The same is with its "God". For if the God is truly supreme and ultimate, this God would understand all the diversity that exist in this world. No jealousy, no war, and no murder would arise.
Note:
As a side note, I believe modern Christianity (Roman Catholicism) is a false church. True Christian teachings lie in Gnosticism as revealed in the Nag Hammadi lost scriptures, where true love, peace, compassion, and humility are taught.