And this is *my* bitterness...because yes, Sam has been told multiple times and by multiple people that he's wrong, and he still goes ahead and does it, and...HE'S WRONG. My complaint here is that Sam apparently *never learns.* He doesn't take anyone else's opinion into consideration, doesn't compromise, doesn't rethink, just goes ahead and does what he thinks is right...and yes, maybe it does make more sense than the others, but well-thought-out still doesn't necessarily mean right. (I once read a 19th-century medical text that was trying to explain why/how people caught tetanus, all without the idea of bacteria. It was very well-thought out, well-reasoned, completely logical and completely wrong, just because they were missing that one little piece of information.)
So to me, if every time Sam's actions turn out to be wrong, then maybe he should think that maybe he's missing some piece of information. Then, the next time three or four people tell him not to do something, he should stop and rethink his plans. Or at least discuss it with them and come up with a compromise. He's supposed to be the smart one, after all.
So if you want to blame the writers for something, it's not for always making him the fall guy (though that's the outcome) but for making him so inflexible and unwilling to listen to others that he just does what he wants, no matter what the consequences. For never learning from his past mistakes.
Now as to consequences: yes, Dean took on the Mark without asking the consequences, but (and I had to go to the SPN Wiki transcript to get this) Cain told him: "But you have to know with the mark comes a great burden. Some would call it a great cost."
Nowhere in there does it imply that he would become a danger to others, or that the burden would impact anyone but himself. So, yes, he was stupid not to ask what exactly that meant, but I can see where he would think that the burden was his (alone) to bear. To me at least, that's not the same as everyone telling Sam how evil the Book was and having him ignore them. And they may have been not completely specific, but there were enough warnings to make Sam want to at least consider the consequences himself. Check the Wiki transcript of "Book of the Damned."
Now, I don't mean to start the "Sam is always wrong and Dean is always right" (or vice versa) fight up again. I think they've *both* been incredibly stupid and stubborn many, many times over the years, and have *both* been excoriated by fans for their wrongdoings (and honestly, judging by the comments and anger on both sides, they're about equal in the blame/fault game.) But I'd just rather point at the writers for constantly making the boys take one small step forward in growth and then 5 or 6 giant steps backwards every time.