A joke is "Take your dog's life and then kill his dog, so it seems". What's the follow-up

Then the killer asked, do you have a girlfriend? The man said no.

Then the killer killed the man, too,

Then he said, single dog is a dog, too.

The above is the second half of the joke.

In fact, there are many versions of this joke, but I didn't find it at the moment and didn't recognize it. This is only the initial version.