I agree. If the condition were that the opponent has too high of a wall, then this card could serve as a deterrence for building. Punishing someone with the theft of their resources for having too little wall is putting too much control in the hands of the person with this card. It's far easier to lower someone's wall than it is to keep your own wall above ten.
While there are several cards that deal additional damage if the wall is low, in those cases at least the recipient is taking damage as a result of not keeping enough of a defense. The punishment fits the crime, so to speak. A card such as this would give direct attack decks a terribly effective way to cripple resources as well as deal damage, and often at that.
In short, it would be too easy to play Night Robbery for its full effect, it would be too difficult to prevent its play, and its effect is not in line with cards that have similar conditions. Make it Rare, limit its efficacy, and/or alter its effect, possibly with a drawback.