Well it seems that killing the parents of family is a right of passage on this show.
Regina kills Leopold and Snow kills Cora, with the repercussion being overall minimal in their now wonderful relationship. Both of these were done with fairly malicious intent and with a complete understanding of the personal ramifications of the murders. By comparison Hook stabbing a witness to his murders and theft to protect himself while still in his villain persona is practically nothing.
However the way things are evolving this will become a bigger sticking point and problem than any of the other past deaths for no other reason than drama.
“The truth isn't easily pinned to a page. In the bathtub of history the truth is harder to hold than the soap and much more difficult to find.” - Terry Pratchett