Can you prove that that is true? Because there's neither biblical nor historical evidence to back this up. The bible is very much in favour of slaughtering heretics, it's mandatory on pain of damnation even (if you 'hold back your sword from blood' (of non-Christians, you're doomed), and history shows Christianity like any religion is rather happy to cause massacres and genocides too.
The Bible contains what you might call "mixed messages".
Reading through the history of the Hebrews in the Old Testament, it is easy to find bloodthirsty passages. However, a Christian should much more reasonably follow the teachings of Christ and his subsequent acolytes, which are to be found in the New Testament. I have no intention of very tediously listing reams of quotes urging non-violent conduct, the pointlessness of conversion by force, peaceful coexistence with fellow humans whether Christian or non-Christian, and so on.
The fact you casually elide all mention of such things for your point, despite your supposed time with Christians, indicates nothing more than cherry-picking, and deliberate dishonesty on your part. I'd guess you're probably not ashamed, as you act like it's more important to appear to be correct than actually be correct.
The OT is certainly part of the Bible, and thus supplies credible (if, by being pre-Christian, I would argue weaker) arguments to defend killing heretics, which have been occasionally enthusiastically used throughout history. However, the vast series of strictures against slaughtering people elsewhere form more than sufficient counter. This sort of thing is because religion, like much in the world, is very complex, and thus ill-suited to the sort of simplistic, cartoon characterisation you wish to apply to it.
If we remove arguments about where the balance lies, simply to point out that it is Biblical to be pacifistic and Biblical to not be, then there are no useful grounds to argue that it is Christian ideology to slaughter people, because it's also Christian ideology to not slaughter people. There is a further problem to arguing that it is Christian ideology to slaughter fellow human beings, because for all that some Christians have, the majority have not and do not now. How the majority of Christians think and act about their religion is, to a great extent, the empirical reality of what Christian ideology believes. Which rather puts you in the position of trying to deny empirical reality. But then, with your prejudice, you may well do so.
Yeah, when it was clear they were going to lose the war and were doing horrible things, after Jews had been getting taken to concentration camps under his nose for years, and after he had approved of a concordat that recognized the nazis as righteous and rightfull rulers....
Pope Pius XI died before WW2 started. Duh.
It is hard to have a useful conversation with someone incognisant and disinterested in the facts important to his argument.
The only thing he criticised, was that nazism was attempting to create a new distribution of powers between religion and nationalism, by creating a new christian national identity that didn't involve the power of the church as a necessary component. A sensible political move for the pope, because the nazis were stripping him of his power over German Christians.
No, it criticised much more. That you argue it does not is just more cherry picking from you. From Wikipedia, because it's easy:
"The encyclical condemned particularly the paganism of the national-socialism ideology, the myth of race and blood, and the fallacy of their conception of God. It warned Catholics that the growing Nazi ideology, which exalted one race over all others, was incompatible with Catholic Christianity"
and you can check the citations for yourself.
Are you never embarrassed at not checking facts, and being so wrong so often? Astonishing.
You see now why I warned that there's a campaign of lies and misinformation going on by Christians to pretend that Hitler and the nazis as a whole weren't faithfull Christians? I said that because it's happening...
This little passage, of course, is just ad hominem conspiracy theory bullshit. It's argument is merely "I don't want to deal with your evidence, so I'm just going to call it a conspiracy of lies."
What I'm arguing is all respected academic biographical/historical analysis that you could read for yourself, had you the intellectual courage and honesty to do so.