You completely missed the point. The differences are irrelevant, it was just an example to explain the logic.
I do believe I have had negative interactions with (some of) them about the things we did disagree on. Do they have any problems with me that are not just thinking I am stupid?
I do not think I am perfect, I just do not think you and those other people are perfectly innocent.
I am not so much saying "collectivism always causes atrocities" as much as I am saying "the atrocities that happen are almost always caused by collectivist ideologies". And I would not say Western imperialism was caused by individualism in the same way things like the holocaust were caused by collectivism.
Sure, let us say these are all because of individualism. They are still in no way as pervasive or horrific as any genocide or war. I am not saying individualism is perfect anyway.
The problem with collectivism is that it often leads to in-group/out-group thinking. This can cause people to favour the in-group and demonize the out-group. Because of prejudices they develop towards the groups they start judging everyone in those groups based on those prejudices which leads to things like racism/sexism/homophobia etc. and in extreme circumstances dehumanization, oppression, and genocide. It is a lot harder for this to happen when you judge people as individuals and do not hold prejudices based on groups or demographics they are in.
I am more specifically against in-group/out-group thinking and I do not like collectivism because it leads to this.