My dog has killed 3 cats

[kimC1]

We have a Staffordshire Bull Terrior/Red Nose Pitbull mix, he was three years old last January. This dog has killed 3 cats. I am at a loss as to why.. He was raised around cats. The last cat he killed, he would not even approach if we near. Due to the fact he had previously killed 2 cats, we were very carefull when ever the cat and dog were near to each other. I always commanded him to be nice and to stay away from the cat. In fact we have two Jack Russell's and he is submissive towards the male. We are responsible pet owners and have never had a dog behave this way. Anyways, I was wondering if we were to have him neutered would this change his behavior?? Thank you!! Kim