This is all well and good when there are good people defending their homes.
However, a wicked person can just shred anyone who comes into their premises, even if they had no intention of committing a crime. Imagine slicing up a group of Jehova's Witnesses, or dicing up some Solicitors.
A wicked person can use this law to argue that the interlopers (to the property, not necessarily the house) had criminal intent, and at that point this law would end up on their side.
Of course, it is up to the police to determine, using Crime Scene Investigation and Forensics, whether 1. there was indeed a crime in progress, or 2. if there was need for excessive force.
"Hey dude, can I borrow a cup of sug-ARGH!"
"Get off my property!" *finishes stabbing*
"Hey you can't do that, you just killed an innocent person!"
"He was in my property. As far as I'm concerned, he's a crook and deserves death!"
See?