Halls of the Dark Muse

I think my new kitty (or newish he is now almost a year old) Strega has decided he wants to be a guard cat. He is normally a very sweet, affectionate kitty who lives attention, and is not given towards being aggressive. I was out in the front yard with him and we were playing around for a bit and then I picked him up, which I know always really irritates him when we are in the middle of playing, but I cannot help myself.

I am holding him, and he is squirming around trying to get away from me when all of the sudden he starts growling. I know the neighbors have cats and they come over to our yard a lot, and some of them he plays with, but others he does not seem to get along with very well, and they like to come into our garage and eat our cats food. I look around and don't see any cats anywhere near by, and there doesn't appear to be anything else around, so I cannot figure out what he is growling at.

Well there happened to be this woman with her kid walking down the sidewalk, and finally I figured out it was actually them he was growling out. He finally leaps out of my arms and gets into full attack mode and crouches down trying to hide behind the tree in the front yard while he stalks this woman and her kid, looking as if he is ready to pounce at any given moment until they finally move out of sight and then he comes bounding back to me ready to play again.