Or is it that because Western society has had some bad actors/states, that we can never thereafter point to bad behaviors or systems of thought?
The question in the thread wasn't whether "islamic terrorism" was compatible with open society, but whether islam itself is.
And my point it, whether it is or not, that "open society" is not as "open" as it pretends to be either.
If some family had done "a bunch of things" (of enslaving, torturing and murderous nature) to your family "over the past thousand years", would you see them as friends? Especially if they continued those things in one way or another up to today, while celebrating how "open" and "tolerant" they are, (as opposed to you, who hates them).
They were "westerners" only in their nationality not in their ethnicity. For some western societies it's difficult to imagine the difference, but it exists -- not everybody gets assimilated to the culture he lives in, especially if there's another culture he feels he belongs to by inheritance.
For those of people (who are not the majority of ISIS, but a small minority) it's about a twisted idea of revenge for what happened and happens to their (origin) countries and to people of the same faith there, and how the west views and treats them -- not about struggling with poverty.
Besides, is it just terrorism that's incompatible with an "open society", or other acts of murder and oppression too? Because from the extermination of 6 million jews down to colonialism, slavery, the pogroms, nukes on civilians, gulags, the massacre in Manchuria, 2 world wars, etc, most western and asian countries don't look that good either.