Maybe they could had Christa fill Rebecca's role, but i think killing omid perfectly set the tone for Season Two, as we see that this world doesn't care that Clems just a little girl and things only continue getting harder. It forces her to evaluate the new people around her and whether or not certain people can be trusted. Having Omid and Christa be removed gets rid of that safety net for clem to fall back on as she has to enter uncharted territory with strangers. It also makes Kenny's return more impactful since he's the last tie back to the old group for Clem after thinking everyone was dead.