Why do people cite Timothy McVeigh as an example of a "Christian terrorist"?

[Starbucks Union Thug HokeyPuck]

Probably because of this letter from 1994

"I know in my heart that I am right in my struggle, Steve. I have come to peace with myself, my God and my cause. Blood will flow in the streets, Steve. Good vs. Evil. Free Men vs. Socialist Wannabe Slaves. Pray it is not your blood, my friend."

...and in a 1996 interview, McVeigh professed belief in "a God", although he said he had "sort of lost touch with" Catholicism and "I never really picked it up, however I do maintain core beliefs."

However, he professed that science was his religion and described himself as an agnostic in the years leading up to his execution.  

Any media attention that was given to his religious beliefs, not unlikely considering he brought up Waco, probably gave some focus to this conversion.  It's possible that gave people the impression he was a religious extremist at the time of the bombing.  

That all said... the guy was a psychopath who's actions are completely incompatible with Christianity or the values of a vast majority of atheists/agnostics.

[Starbucks Union Thug HokeyPuck]

I've not really heard of him described as such. The only ones who come to mind are those who murder doctors at abortion clinics or the 1996 Atlanta bomber, Eric Rudolph.

Don't forget the Norwegian guy.  He was a wacko.