I don't agree with opinions representing the holder's truth, at least not all the time. For most people, some of their opinions are more like part of their identity. Identifying with that particular belief lets them belong to some group or tribe, or lets them see themselves in a certain way. They see these beliefs as axiomatic, rather than a fact-based opinion (not that most people would ever admit this).
Attacking that belief, in their eyes, is tantamount to attacking them, because they see it as an attack on part of who they are. And to change that belief, part of them has to die. This can be useful to keep in mind when someone comes along who believes odd or even reprehensible things even in the face of bullet-proof logic and great emotional rhetoric.