My JRs seem very territorial

Recently my neighbour let a stray JR into our house thinking it was our JR. We let it outside to play with our male and female. All was going well for the 1st bit. Then my dogs went in for the kill. My male had it pinned down on its back with his teeth firmly planted around its neck. The stray seemed paralyzed and unable to move. My female was standing by barking at it. My male did draw blood from his bite before we intervened. Is this just a normal "get off my lawn" fight or can JRs become so terratorial that they will kill for it?