If you edit my post, please note that it has been edited.
As to the halacha, shok is NOT clearly defined and people showing parts of their knees may have Rabbinic tradition backing them up. I say may because not all communities hold by this.
As to comparing tznius with murder – tznius is between a person and Hashem. Murder is between a person and another person. I would compare murder more closely with tax evasion than tznius.
We should really question why women are getting close enough to notice the difference between the few inches on the knees.
EDITED