Can you understand why religion has killed so many people throughout history?

You need to also understand that 1,000 years ago, the religion was as much, if not more, in many cases a better governing body than the government. And yes, there were/are evil people who did/do stupid things to others while believing and claiming they are doing it with the will of God. Legitimate totalitarian regimes in the 1900s killed 100+ million people. Evil existed then, it exists now.