Doing the human thing, letting Nicholas live will bite them on the ass.
From what I've read the odds of making grenade to explode by shooting it are low, though it depends on the caliber of bullet.
They didn't let Aiden die, because even if they'd gotten him off those spikes, he'd probably have bled to death. (i didn't think about the call back to that season 2 kid...Randall?)
Beth and Tyreese's deaths seem like they were for nothing now, freeing Noah and trying to get him home. But that's the point...never help anyone.
The Alexandrians are right. Okay, they aren't...their "what happens outside the walls stays there" attitude make me wonder how many people they've gone through.