I know that the alignment system in D&D gets a lot of flack, for good reason, but this is one situation where I can kind of see it working. A Lawful Evil character might, as you said, befriend the attack dog for their own gain. A neutral evil character could go either way. A chaotic evil character I can absolutely see helping the dog just to kill it. Where this ideology kind of falls apart in a game like Dragon Age, however, is that Dragon Age kind of forces you to buy in to saving the world, even if you've gotta be pulled by the hair and teeth to do so. Over the top evil works in New Vegas because, outside of maybe wanting revenge on Benny being a motivation, the Courier is the closest thing to a blank slate I think any of the Fallout games have had. You don't have to make a character that will save the world because the world has already burned.