You ask a question, but your use of the word "really" indicates that you have a source that is saying this. Minus the word "really", one could either answer "no" or yes" and be done with your question. I answer "no", minus the "really".

Christians follow Jesus and place their faith in in the belief that Jesus rose from the dead and ascended into heave after his murder at the hands of the state and by way of collusion of worldly power. Such worldly power is considered to be wielded by satan, where death and pain and public humiliation are not the end and not even to be feared, but instead to be expected as we seek His Kingdom which is not a kingdom of these worldly powers. So, rather than believe in murdering non-believers, Christians would praise God and pray for non-believers as the non-believers murder them-- not that those are always the only two choices given.

Some NT verses indicate that it would be better for someone to die, perhaps (edited to add that word and say I don't have the specific verses in mind) even to kill the person, than let them sin and break their connection to Sanctification and with God and redemption. But, I have never read those as saying to do that, but just that it emphasizes the gravity of the circumstance. Salvation is about our souls, not our earthly lives at present.

There is a lot of killing in the old testament. I try to separate out when God is actually saying it is a good thing, or when he is testing people, like when Abraham is tested to see if he would kill his own son for God. As bad as that event is, it is important to recognize a couple of things.

One is that the worship of Moloch and his consort Ashteroth involved a lot of sex rituals and the murder of children. Child sacrifice was common among some people. Some think the Etruscans and the Carthaginians brought the Phoenician practice of child sacrifice to the western Mediterranean as groups of them figured they could start a colony away from neighbors who attacked them for that practice. The OT is especially harsh in dealing with this practice, and other civilizations in the area who were cruel conquerors are not condemned as this practice is. (the other thing worth remembering is tangential to this topic, but it is the simple fact that God sacrificed his Word made flesh, doing what Abraham only had to be willing to do. Mary suffered what Abraham did not. Israel did not get a military leader to overthrow Rome and show the iron age world who's boss. We got nothing but an assurance that glory or happiness or comfort or accolades in this life are not the point ever.)

This ties into the specific groups that the bible describes God telling the Israelites to wipe out, which sounds really bad but Michael Heiser makes the case that these are all groups associated with the Nephilim. If the Nephilim were wiped out, then we have no real context to understand such campaigns today. This also presumes the Nephilim were real. But if one presumes they were not, then one could equally presume the stories are not real. It seems difficult to presume the stories real but the Nephilim not real. Archaeological evidence of such destruction might be sparse, but that is often the case and needles do occasionally end up in haystacks regardless of how tough they are to find.

So, I don't know in what context you are using "really", but really Christians do not believe in murdering non-believers. The Crusades were against a force that was aggressively attacking Christendom, and also involved east-west Christian fighting which still weighs heavily on attempts to heal the schism between them. Much bad happened in the Crusades, but they are more complicated than currently believed in popular culture and not completely unjustified as one look at how Islam deals with women or non-believers would show. The Spanish Inquisition and other such events, like how Jan Hus was dealt with, I think are big problems. I do not like the idea that the church can pass some decree or sentence and the civil authorities carrying out are the ones doing the killing. It is a repetition, especially typologically, of the tag-team between temple priests and Roman authority condemning Christ.

People often do bad things, even Christians and even those who claim to act in the name of God or the Church. A big part of Catholicism is accepting that "the Church" has often had very very bad leaders not just now but across history. Such bad leaders are meant, by the worldly powers, to make us see the those bad leaders as if they were the Church. This has lead to a lot of division and lot of problems that have been seized upon by wordly powers. The biggest difference between Jan Hus and Martin Luther is that political leaders saw a chance for increased freedom and centralized power in Luther where they didn't catch the same opening when Hus was alive. It is probably more complicated than that, but that is a factor. You go from questioning "works" to beheading women who won't give Henry the VIII a son and to his daughter being advised by a guy, John Dee, who was talking to fallen angels that wanted his partner's elderly wife to have sex with John or some weird voyeuristic trip that came through their "scrying" sessions.