User blog comment:Rafe Adler/"The Rising Son" Discussion/@comment-26982641-20171020105611/@comment-1031724-20171022223029

Sam tried to kill Samhain with the demon knife, but screwed that up, which I think was intentional so he'd have an excuse to use his powers. At which point, as noted above, he could only exorcise demons and send them back to hell. So, he didn't kill him like he did Alastair and Lilith.