Yeah this is a thing I've really been noticing, because I've been keeping track of who actually takes the side of the monster of the week in the "not all monsters" debate, and the pattern that emerges is actually incredibly interesting.
Dean's situation is complex, because it becomes obvious he's caught between a clear ethical code he has developed on his own that requires him to weigh the actual merits of the individual case in front of him, and the lessons he had drilled into his head from early childhood that All Monsters Are Bad. he's got these two contradictory voices pulling him in two different directions, but in the episodes where he's the "not all monsters" guy, there's a pretty clear pattern to it. Dean tends to side with monsters who haven't hurt anybody, whether because they're children (Jesse, Bobby John, Emma, etc) or whether it's because they just happen to be a supernatural being who got caught in the crossfire of somebody else's problems (Andy, the rest of the coven in Malleus Maleficarum, etc), that's usually his motivation.
It's not based on the actual biological status of the "monster" in question but more on an assessment of their actions (i.e. humans are monsters if they're killing a bunch of people, non-humans are not monsters if they've never hurt anybody). And of course this isn't perfectly consistent, because as I said he's also clearly got John's voice nagging in the back of his head about the whole thing and that complicates and clouds his ability to be objective. he's caught between a consequentialist ethic he's built for himself based on his own reasoning and morality, and a deontological education from his father. however, for the most part, you can reasonably expect him to default to his own code of ethics if a case comes up that challenges John's deontological view of monsters.
Sam, on the other hand... he's much more likely to say "not all monsters" based, not on any actual ethical code he's developed or a rational assessment of the case in question, but more based on whether he personally relates to the monster in question. Whether it's defending Max in Nightmare because of their shared backstory, championing the plight of ghosts to Molly in Roadkill in a way that's transparently more about him and his own struggle with feeling like his psychic situation is beyond his control than it is about anything he's ever expressed about ghosts in general before or since, or desperately trying to help Jack (rugarou Jack, not nephil Jack) avoid succumbing to his monstrous appetite in Metamorphosis because he's projecting and wants to believe he himself can be saved, this is Sam's general pattern.
Sam wants to save the monster, not because Sam has evaluated the situation and believes there's merit to an argument that this particular case doesn't warrant a bloody end, but because Sam personally relates to the monster of the week for whatever reason. Sam sees himself in the monstrous at times and empathizes, which is a good thing broadly speaking, but it's fundamentally about Sam more than it is about the subject of the episode. and it can run in the other direction, too: this is why Sam comes down so hard on Andy in Simon Said and is so ready to believe that he's absolutely definitely 100% a murderer— Sam at that moment is in a particularly dark place of fearing what's happening to him, and sees himself in Andy. As a result he's very inclined to play judge, jury, and executioner and has to be talked down by Dean.
It's ultimately not actually about the monster of the week, with Sam. It's about what Sam sees of himself in them, and how he subsequently judges himself-in-the-Other depends entirely on how he feels about himself at that point in time. it leads to Sam coming down on the side of the monster of the week somewhat more often than Dean does, because Sam actively and personally relates to their situation more frequently than Dean does, but it does not actually provide him with any kind of coherent ethical framework with which to evaluate the cases they're working on.
Basically what I'm getting at is that it's all vibes-based with Sam.
pleaseburger
