This is curious. I can understand why you don't want the perpetrator to be in a group you identify with, but why does it happen when group identification isn't the issue? There's so much space between conservatives and any militia that would bomb a parade, why should they care?
Why? Because... it's the narrative! Who is going to win the narrative, who is going to get the better narrative out of the event. If it was Muslim terrorists, the conservatives can say that we shouldn't have all these Muslim immigrants, and look at what the tolerance gets us. Or maybe they say something else--I really can't speak for the conservatives because I've never been one.
If it was a militia type/white supremacist, the liberals could talk about how the subtle racism in conservatives (dehumanizing welfare recipients, bashing government programs) fosters the much more virulent, violent distrust of government, blah, blah, blah.
I have an idea. Let's not score points over terrorism. Let's not be gleeful that the other side will have mud on their faces. By all means we should talk about how it happened, what sort of preventive steps we could take. But let's refrain from hoping a certain group is responsible. I wish none of them had executed those acts. No narrative is worth those lives and limbs.
Extras. Parade of yech: