It's like a mix of Hamlet and Macbeth, which makes sense considering it's the source of Hamlet. Unlike Hamlet, the lead witnesses his uncle kill his father so he doesn't need a ghost to tell him, but he thinks his mother has been taken unwillingly until she reveals she hated his father and got the uncle, who she truly loves, to kill him - and wanted her son murdered as well, because unlike her new kid he wasn't a product of love. So it's kind of like Hamlet but if Lady Macbeth was Gertrude and the witches turned up.
I think the mother's thank you for being stabbed through the heart was that it was appropriate for her to be killed that way, maybe it gave her the equivalent of dying in battle, so a noble warrior's death. And the director has said that the final fight had to be them naked, because Vikings saw that as a sign of strength and manliness, but decided it would be too distracting to have the dicks swinging around so the actors fought in loin cloths but they had to CGI in hints of swinging balls so they didn't look artificially bare. Skarsgaard, at least, has no problems going naked, so that wasn't a concern, it was just a choice to keep the attention on the big swords in shot, not the little swords dangling about.
