Given that it's yet another episode where Andy is in danger, I think it's going to turn out that the rookie who was killed, who's case Luke is obsessed with turned out to have been Sam's partner. That's why he didn't want a partner, and definitely not another rookie woman who he might lose. That's also why he's so hard on Andy and worried about her all the time, and now that he's her TO makes sure he's really teaching her how to do the job well.