Off the top of my head I can think of a few pairs like that. Regina Mills killed Graham Hunt, Jeremy Gilbert killed Kol Mikaelson, Guy of Gisborne killed Lady Marian, Klaus Mikaelson killed Jenna Sommers. Jaina killed Jacen
Ignoring is probably the best policy. Until they cross a line like Moriarty has, repeatedly, there's no point getting worked up about things.