the islamic theocracy al Qaeda wants defines shia as apostates - killing shia is right up there in the list with Israelis Americans Brits and misc infidels
This gives foreign-directed al Qaeda common cause with many Iraq sunnis, especially holdouts from the privileged clans that benefitted from blood or other alliance ties to Saddam and his security apparatus
Check back to the various al Qaeda manifestos released via video and in full text and you will see and hear them discussing their goals for destroying the shia