While I don't condone what he did, or any sort of animal cruelty, I would be interested in hearing what the roommate has to say about it. He might be an unrepentant arsehole, or he could've been having a bad day, lost his temper, and now feels really guilty about it. We have no idea, because all we have to go on are a couple of sentences, on the basis of which Judge Internet has already convicted him.
Like I, and others, have said, what's wrong with talking to the man? If it turns out he thinks what he did was okay, then by all means report him to the authorities and try and get the dog rescued. But to advocate just stealing his pet without even making an effort to hear his side of the story doesn't seem like the best advice. Like my mother always said, two wrongs don't make a right.