In grand theft auto 4 does Michelle know that you kill people?

Answer 1

Yes she does. But I don't know how far you are into the game, so: SPOILER ALERT..........

Because she works for the government, she has plenty of surveillance information on Niko. She does actually say to him "you've killed people!" later on in the game as their relationship finishes and Niko has to start doing jobs for the government.