NSFW Speculation in this post - skip if not interesting in wounds and injuries
Boxers do it all the time (punch the liver - on purpose).
As do street fighters and thugs who want to kill the person with their fists. Kicking the liver area is not uncommon in beatings (but surprisingly, not as fatal as one would think - a knife is the lethal choice).
I think you are viewing BK very different than I am. This is a person who had been studying crime and criminals for a few years. This is a person who knew one martial art (kickboxing; sometimes said to have done boxing). This is a person who planned and practiced. And, as I keep saying, if you go google how to kill someone with a knife, there are instructions. Why would people bother to write out how to find and stab the liver if no one ever did it?
My point is that when he entered the house, just as when he entered the classroom, he was pugilistic, amped up but from his own POV, analytic and calculated
. Just as in his favorite theory of crime (Rational Choice Theory - not designed for use in criminology, really, but used nonetheless and mentioned by the professor who thought he was brilliant as a theory he liked to use; that and Script Theory - where the criminal carefully scripts out the crime).
You are absolutely right that if he stabbed (hoping to avoid bone), it would be very easy to cause liver and lung damage - with one or two gashes. I'm pretty sure the families have already heard all about this and if not, it's because they know how to restrict media that discusses crime (there are many other places on the web without the rules we have here).
No mention of any other organs besides the liver and lungs, so far, which is why I'm going with that theory. Just my own speculation. It's not that hard to learn anatomy (although we have no evidence that DeSales or WSU put any such courses in their Crim Just curriculum - it does come up). Interestingly, in the jail records in my county, insertion of an object (shiv or whatever) into the liver was one of the more common ways to attempt suicide or homicide inside jail - until about 1980,when metal detectors and other measures came into place. So criminals figured it out and even did it to themselves. I know it's hard (and awful) to picture.
I am not saying he is smart, just that he used what he thought as Scripting and Rational Choice to develop a crime plan. He isn't the first to do it.