We all agree that critical wounds are not juicy enough for monsters, don't we? They are a bit of lackluster and fitting more for PCs.
On the other hand, rules system is lacking the surprise element of going for instant kill. The cruel brutal world of Warhammer should be all about finishing your opponent with a single blow, even the mightiest of enemies could fall with one well-placed arrow.
I've combined critical wounds on monsters and instant kill to one simple house rule. See my house rules here. Page 6.
Each time a monster suffers one or more critical wounds it must perform a Resilience test, difficulty based on number of critical wounds suffered. Player may use fortune points to add misfortune to this check and using Chaos Points (ACE-replacement) house rule, GM may balance the odds as well.
Failure = instant kill (with brutal narrative!)
each Base = +1 wound
I think the extra check is justified as the reward is so great and adds to the excitement of the battle with some player interaction of using fortune points.
Our group is not excited about critical wounds and never even remember to go after these .. with instant kill, they will sure get interested!