He's lucky it didn't gut him like a fish.

He, or someone else nearby, has probably been feeding it. If you feed them, they come to expect food and when that is not provided, they get aggressive. Don't feed the wildlife.

The kangaroo might have attacked the dogs thinking they were dingos. Kangaroos don't like dingos and will try to drown them in ponds.

Why didn't he just shoot it?

imagine fighting a rat the size of a human.