So this was just your average everyday Iraqi disagreement, not jihadi terrorism.
I remember the stories of thallium poisoning during the Saddam years. Gruesome. I believe it began with the KGB, who used it on particularly obstreperous exiled dissidents they wanted to make an example of. That and ricin.
You may be right, an maybe not. "Former Offical" indicates he was probably Sunni. The jihadi's have been allied most closely with Sunni Iraqi factions....until recently when a large number of Sunni warlords have switched to backing the coalition against al Qaeda in Iraq. But this does not mean every Sunni has switched sides.