Agent Todd was killed off ostensibly because Sasha Alexander wanted to leave the show to pursue other opportunities. However, the producer surely could have just decided to transfer Todd or disable her or create some family emergency to take her away, or something short of death. That would have left the door open for a return in the future. I have to wonder if Alexander's desire to leave didn't rankle the producers.
I remember reading comments of J. Michael Straczynski, creator and writer of Babylon 5 when he removed the character Talia Winters, that he had a trap door built underneath every one of his characters. The actress Andrea Thompson wanted more screen time and to be a more significant player in the drama. However, Straczynski was producing a novel for television and Andrea was not going to get her wishes. So Straczynski pulled the plug. He said at one point, "Generally speaking, about once a year, toward the end of the year, I kinda look around at the characters with a loaded gun in my hand, and say, "Hmmm...if I take out *that* person, what happens? Is there anyone here I can afford to lose? Would it be more dramatically interesting to have this person alive, or dead? What is the absolute bare minimum of characters I need to get to the end of the story and achieve what I have to achieve?"
I don't know if Bellasario and his fellow producers took that approach to NCIS, but this program is episodic by nature. There does not seem to be an overall story arc for the series. Therefore, I don't understand the compelling reason to get rid of Agent Todd, especially after all the crap she had to put up with from DiNozzo all season long. It just seems really unjust, considering this is a show about a team that seeks justice.
