I used to have a pair of greyhounds, both from off the track. They were kneejerk-efficient cat killers. It must be because they are bred to try to catch little furry things. When there is suddenly a cat in front of them that doesn't have the experience or sense to know its danger, the outcome is foreordained.

I don't know if whippets have the same psychological bent but I would NOT trust them within eyesight of any cat I prized.