This discussion may contain spoilers!
Yo so originally Sam the dog was meant to be either set free or killed, instead of killed or left to die. I say this because the choice is called "mercy" the dictionary definition for mercy in case you didn't is compassion or forgiveness shown towards someone whom it is within one's power to punish or harm. Also when Luke and Pete show up, Clem only has the option to say I killed the dog (in explaining his fate) she doesn't get to say i left him to die or put him out of his misery, because it was never meant to happen! it was meant to be that u either let the dog go, or killed it. This is why Luke gets so upset!!! telltale re wrote it so that Clem actually does the right thing by killing sam, instead of leaving him to suffer!! does this make sense?? That would of been so much better....coz if you don't kill sam when he's already going to die/suffering ur just awful. but if sam had been ok and u have to choose after he attacked u, it would of been harder for some. they probably rewrote it like this because it could of been seen as animal cruelty to kill an alive/okay dog