Alcohol is an illegal drug in Iraq.
Are you so sure. If Iraq were under Sharia law it would be, but Iraq was a secular state prior to the coalition invasion. I haven't seen any story stating that the occupying forces had outlawed alcohol. According to lots of news stories I have seen on Free Republic, the booze flowed rather freely in Saddam Hussein's Iraq if you had the money to buy it.
Are you sure they were killed for being Christians?
Or were they killed for ignoring several warnings to shut down their drug business?
The de facto law is Islamic law, the drug dealers were warned several times by the local militia, and they paid the price for selling drugs.
Being a Christian is not the same as selling booze.