Hi Robin,
It seems to me that when you say "one cannot be a Christian and an evolutionist" that you are using your view of what it means to be a Christian. In your view, a Christian does not believe in a God who "created a situation in which, in order to survive, life forms must torture, kill, and eat other life forms".
I think there are many people who don't share your view of what it means to be a Christian. There are other very common views of Chrisianity in which it is not considered contradictory to accept evolution. Catholocism and the Methodists come to mind.
--Percy