This is the second posting on the 1st law. Are YW235-236 that hard to understand? I thought they provided reasonable guidance for games. Intent matters, accidental killing counts, wizards using mundane force don't count, and every gaming group should discuss it. Good advice.
In terms of booby-trapped wards, I think its pretty clear that both the intent and accident part would make it a violation. If the wizard is intending to create a ward that can take out a red court vamp (for instance), that's a pretty clear intent of lethal force. If a mortal gets caught in it, you've broken the law. If you create a ward with the power of a bug zapper and it happens to trip someone's pacemaker, that is like a conjured wind accidentally sweeping someone off a roof - you didn't intend it, the force wasn't lethal, but the accident was still killing with magic. But I would also say that the fourth part of my summary trumps the other three - discuss it with your group and come to some decision about it. If you all decide that for your game it doesn't apply, that's fine too.