There's really no acceptable situation for murdering dozens of school children. I feel like this doesn't really need explanation.
Imagine this, Ted Bundy killed 100s of women. Then you kill dozens of school children. are you good? did you do a good thing? are you a good person? No, it doesn't really matter what Ted Bundy did. Why would you think that matters?
The burden of not killing dozens of school children remains intact.