Why was there no explination for the departure of moira kelly in season 1, it just seems strange to me why they have never aknowledged this.

If there had been an explanation, then they would have had to explain it. The real situation is that Aaron and Moira came to the conclusion that her character wasn't working out, and it was decided to write her out of the show.

I think they should have killed her at Roslyn, but she wasn't at the Newseum.

They did that a lot.

They left a lot of things for us to try to figure out what happened, why they did things, what they were thinking or feeling...