Beheading by the government is appropriate. Beheadings by terrorist groups or insurrectionists is not. One is sanctioned by God and the others are criminals to be destroyed by a legitimate government. One is done in public under government authority and the other in hiding because they know they're evil.
For God's sake we even put down killer Pit Bulls humanely. If you substitute the word "Allah" for God in your reply, it might well have been written by Saddam. His "government" was legitimate. He was elected with 100% of the vote as I recall.