Last Equation – how to stop the infected

I might have on accident given my players an impossible task. Maybe that's good, maybe not, but I wanted to know if anyone knows any way fro them to fix it.

We started The Last Equation last session, and it went great. They've started to understand the danger and wanted to talk to the officers who were first on the scene, first of all with the photographer who documented the numbers.

Now, i hadn't prepared anything for this, but I like rewarding (punishing?) my players when my players think of something outside the box. We're new to DG, so anytime they actively investigate is a good thing. So I made up on the spot that the photographer has a good memory and is a hobbyist mathematician. So they figured out she was looking tired, saw some notes and math books and put 2 and 2 together.

Now they've confiscated her camera, they've destroyed her notes, but since i made a showing of the fact that she has the entire number sequence memorized, they think they are shit out of luck. Most of them don't want to kill her, one of the players is a bit more gung ho than the rest and will probably sneak out to "do what is necessary".

I don't mind this of course, it's part of the game. But our last adventure (Viscid) ended so incredibly bleak (one of the players shot an innocent man in a split second decision to cover up the whole thing). It was a great ending, but I also don't want my players to think that the right choice is always to kill innocents. Basically preserve the morally grey aspect and the dilemmas.

Has anyone dealt with similar things. I want the game to be the right tone, so bleak, but also I don't want my players to feel like it always has to be. I think if every adventure ends like the previous one it will wear thin fast.