I'm afraid I wasn't able to attend this month's meeting, so I did not see the discussion you mention. However, especially since the whole point was to pick controversial topics so that people would disagree, I cannot say I am surprised.
First, just as a disclaimer, let me say that as for the issues you mention I think I have the same opinions as you. I did not support W's invasion of Iraq since I considered it both to be morally unjustified and also to create more problems than it solved. Unfortunately, I think the facts have proved me right...and now we've got to make a decision between many different bad options since there are no good choices left to make.
But, that's beside the point; you ask what I think about the disagreement on this subject at the meeting.
* I think this clearly demonstrates that not all humanists think alike on political questions. I understand why you might think that we would. We share a non-religious worldview along with a desire to help people in need, and a belief that the decisions should be based on rationality and verifiable evidence. However, it turns out that even with all of this in common we can reach different conclusions. That's not something unique to humanists...nor is the feeling that this is a surprise. I mean, I think there are a lot of people who were strongly opposed to the invasion of Iraq and felt that the source of this feeling was their belief in God and the Bible. These religious people were just as surprised as you to realize that there were people of their faith using the same religion as an argument FOR the invasion!
* Disagreements like this should give us pause to consider whether the other viewpoint has some good arguments that we're unfairly discounted. A common mistake that all people make in situations where all of the choices are bad (as I've said above I think we currently have in Iraq) is to point out the negatives in an opponents argument without acknowledging the negatives in their own. Your description of someone else's argument lists what is wrong with it (genocide, presumption of guilt, etc.) but I would guess that the other person would similarly say what is wrong with your viewpoint. You don't necessarily have to change your mind, but an open discussion such as the one the SHL held is a good opportunity to make sure that you've actually thought through the negative side of your own viewpoint instead of just focusing on what is wrong with the others.
* Remember, I was not there and so did not actually hear what anyone said. It could be that I would agree with you that in the end the viewpoint expressed by the person supporting the administration's policies was completely illogical and immoral. If so, then there is still something interesting to consider: what is it that makes them believe it? Quite often, in diverse situations and not just the one that we find ourselves in now, I see people making illogical decisions when one particular emotion is so strong as to drown out reason. Fear and jealousy are two emotions that often do this, and perhaps an overdeveloped fear of terrorism is a factor in the continued support for Bush's failed military exploits. But, I would actually put more of the blame on "anger" and "pride". People are still so angry about the events of 9/11/2001 that they can applaud the death they see in Iraq as "our revenge" without even noticing that Iraqis had nothing to do with 9/11...and patriotism seems to be able to keep people from recognizing that there are wars we cannot win or that torture is wrong even when "the good guys" do it.
BTW Was there anything else discussed at the meeting of interest?