Is that widely accepted as fact, as opposed to an opinion? My understanding was that 'we' were too eager to depose Saddam and accepted the evidence of WMDs without enough investigation, but that's very different to the claim that 'we' knew.
Do you mean that Bin Laden was against Al Qaeda in Iraq...
Really? At best, they thought Saddam had WMDs and wanted to make the region safe, at worst they wanted to topple a leader they didn't like and impose their own ideals of a democracy on Iraq. I've no idea what was in their minds, but surely it's not as bad as just killing as many non believing...