Since a boobytrap is an emplaced device, and is therefore targeted indiscrimiinately, it is illegal to use.
Think about it - Emergency Services comes in because your wife called 911 while you were at work, and forgot to disarm the "alarums." Who's liable for the EMT lying in the doorway? Besides, now the other EMT's are busy seeing to their associate while your wife is having trouble - and the rest of the EMT's are having a hard time coming in.
Since a boobytrap is a "dumb" device, it is not specifically targeted and can't be relied upon to be used effectively.
If there were a guaranteeed way to make a "smart" boobytrap - GUARANTEED - it would probably be legal. Meanwhile, work with non-lethal means. I suggest checking out www.stopthecrime.net (or .com?) for more info along these lines.
FFZ