whether the knife amounts to "offensive weapon" depends on the intention.  With only 1 line of words without appreciate the evidence of the whole case, I don't think any lawyer can possibly tell whether the knife can be regarded as "offensive weapon".

"但今次我見唔到如何入罪, 一百個民居有九十九個一定搵到刀, 所以他在屋企內持刀絕對係合理。"

Again, this is quite wrong.  The real test is, the intention of using a particular object.  If the intention is to use it to cause injury to others, anything can be offensive weapon.