bad for the 100 or so non murderous wastelanders
good for the enclave and humanity as a whole
"humanity as a whole," not really
that highlighted bit is the river they were going to poison
that's it
The needs of the many outweigh the needs of the few
right. so, rather than killing a bunch of people, which isn't going to accomplish anything, purify the water, so that civilization can start to recover, with easier access to clean water and food