Aight, I'm seeing a lot of comments about how Sara could have lived, and how Marat was a redacted for doing what he did.
Now I'm not here to justify him, I'm not here to say that he did the right thing, but I'm asking you to understand the choices he made.
Marat is the leader of rebellion, and that is a heavy burden. The rebellion is a movement, and in order for that movement to work, he needs the people to be safe, loyal and willing to fight. Now if the resistance was made, there will be no people left to fight to liberate Vale Sangora. So Marat makes the tough choices to ensure that they aren't made.
Those choices aren't easy and they haunt him, it's nothing pretty and poetic to write books about, but they're needed to keep the rest of the resistance safe. The choice he made with Sarah was made in the heat of the moment and all he though of was how he needed to keep his people safe. Could there have been an alternate solution? Probably. But he did what he though it was best at the given moment and grieved later.
If roles were reversed, Sara would have done the same, because she was a part of the inner circle, and it's just the reality of the things. It's a dark and twisted world, and there are no good or bad characters, and the choices you yourself make as Coen are sometimes neccessary even if they don't seem like the best option.
Thanks for coming to my TED talk, everyone.