If Katie knew (Thought) DMac is alive she would've been looking for him, ultimately making things worse.What was the purpose of getting Kate to think they killed Dmac?
Katie would've encouraged DMac's relationship with that girl of course that would've lead to DMac being more in the streets and it's already established that DMac is a magnet for trouble sooner or later 5-0 would've found out DMac popped that cop that would've brought more heat on Tommy and his fruity booty brother.