His Christianity.
Bingo, SkyPilot!
When I saw the question, that was the answer that immediately came to mind.
We are involved in great spiritual warfare, and George W. Bush represents to the Godless American left, the enemy.........righteousness, justice and most of all LIFE. That, IMO, is why they really hate him.
(I can't explain the freeper haters.......they make no sense at all......just mad at the world, I guess).