I generally agree with the first answer, although I would point out that the first time Montag sees the Hound it is "asleep." So don't write that it is walking around and it snarls at him.
I think I would add a couple of things to the first answer. First of all, the Hound has 8 legs and that makes it seem like a spider. Spiders are scarier to most people than dogs -- their body shape and the way they move is.
Also, look at the exact words Bradbury uses to describe this thing. His first words about it are
The Mechanical Hound slept but did not sleep, lived but did not live...
So right there, it already sounds evil -- like a vampire or something. Then he goes on to say
It was like a great bee come home from some field where the honey is full of poison wildness, of insanity and nightmare, its body crammed with that over-rich nectar and now it was sleeping the evil out of itself.
To me, this makes it seem evil.
Finally, in terms of destructiveness, I would look at that game they play with it where it runs around killing all the animals. That just shows it destroying everything it is asked to.