I've noticed people often point to Donatello as the model soulless person and then argue the sharp contrast between him and Sam as proof that deep down, Sam is just a really shitty guy who has terrible thoughts that he manages to keep under wraps only with a soul. I don't think this is that fair of a take.
I think using Donatello as the contrasting character is cherry-picking and that Donatello is a very poor comparison for Sam. Donatello is more of an exceptional soulless person than a rule. We see multiple examples in season 11 of other people losing their souls. Several go crazy and murder people like feral animals. None of them have the ability to feel right vs wrong—they just understanding the existence of the societal rules they've grown up with, and either accept those rules or don't based on what they believe is in their best interest. Previous traumas also seem to play a role.
Someone like Donatello has never been through anything traumatic that we know of, and he's just a professor. He has no need or incentive to kill anyone and no previous traumas that might induce him to want to harm anybody. Intellectually, he likely understands rules as a good way of maintaining societal order. He also understands that if he breaks the rules, he's going to get in trouble and lose his job, go to jail, etc. He isn't a fighter. He's just a professor and all he wants to be is a professor. The biggest moral quandary he deals with on a daily basis is whether to bump a student's course total up half a point to get them to the next letter grade. His goal is simply to continue being a professor. The rational option for him is to be a model citizen whether he can feel what the right thing is or only understands rules on an intellectual level.
The core thing soulless Sam tells you about Sam is that being a hunter is the occupation he finds most interesting (or else he'd go do something else that he found more intellectually fulfilling). All he cares about is killing monsters and capturing alphas and the intellectual fulfillment he feels when he clocks a witnesses lies (6.06) or solves the latest case puzzle. Fulfillment isn't nearly so cut and dry for Sam with a soul, because he has to deal with emotions which create more conflicting goals and desires than simple intellectual stimulation.
Hunters live lives where they are constantly faced with moral dilemmas that normal people will never face, and they know how to escape legal consequences. This is what makes soulless Sam such a dangerous hunter and why the outcome is so different from someone like Donatello. Soulless Sam's most rational option is not necessarily the societally acceptable one. Sometimes there is no societally acceptable option or any written rule that encompasses the complexity of the actual situation. Normally, hunters will "feel" out what's right and what's wrong in these situations. Soulless Sam identifies this ability as something he lacks. Sam recognizes this as a hindrance at first and wants Dean to fill that role for him (6.01, 6.06, 6.08). However, he also slowly begins to think that maybe other hunters are the problem and are hindered by their emotions and that he is better because he's capable of pure efficiency-based rationality. This is why he lies to Samuel, lies to Dean, and keeps secrets from Dean for Samuel (6.06, 6.07). He wants their help to reach his own goals, but increasingly sees their potential emotional reactions to his actions and each others actions as an inefficient hindrance that will impede the mission.
Letting a vamp turn Dean isn't something Sam with a soul would do or likely even think to do. This is the guy who went on two multi month revenge quests after his brother was killed (3.11, 4.01/4.09). With a soul, he cares about Dean's safety—even when he pretends he doesn't. He's hurt and killed people for hurting Dean or to keep Dean safe and he's been willing to hurt and kill more (example 1, example 2, example 3, example 4, example 5, example 6). Soulless Sam also has no reason to want Dean harmed unless it benefits him. There is nothing in him to love Dean, but there's also nothing in him to hate Dean. When he sees Dean being turned, and stops in his tracks, and smiles, it isn't actually because he's taking pleasure in seeing Dean being humiliated. There's nothing in him to feel hate toward Dean like that. Even hate like that would require a soul. Why Sam is smiling—what he's taking pleasure in—is seeing his plan come together—a solution to the issue of "find the alpha vampire". He immediately realizes he has an "in", and because Dean is merely a tool who can benefit him, his suffering that occurs in the process doesn't matter to the equation—just what Soulless Sam can get out of him. Using Dean, he's solved the latest intellectual puzzle. The same thing happens in 6.10 when Meg is on Dean's lap with a knife, making sexually suggestive commentary. Sam laughs—and Dean at first thinks it's because soulless Sam enjoys seeing him being treated that way, but Sam tells us he's laughing because he's figured Meg out—because he realizes she's desperate and scared of Crowley. Laughing at her takes also takes away her sense that she has the upper hand.
