well, that's nice.
I like her, she definitely could have been written better in the comics, and she could have been dressed better for sure. But after actually getting a hold of some of the comics with her in it. I can see she, and her relationship with Loki, is more layered, and more complicated than I first thought. Some not nice things in there, for sure, in the very early stuff, but in the later comics the love developed both ways and became surprisingly tender and genuine at times, actually. And she is much stronger and more powerful with sorcery than I first thought. (to be able to make an almost exact duplicate of a person, and to even attempt to break one of Odin's spells, not to mention binding Loki's spirit to a totally kick ass suit of armor) So, no, I dont believe she is a victim or a wimp, neither is she a villain. That is clear. It is a complex relationship for sure though, and one I'd love to see explored in the MCU.
She did actually "fess up" to that. I know because I own the comic where that happened. She fessed up in defense of her husband, and in a great speech actually and that was both showing her bravery at at the same time was clearly very remorseful. Loki comes out of the armour to her defense and to hold her, comfort her, when he sees how upset she is, and that Thor is angry.
Even Thor understands it was an accident, and out of love for Loki, and Thor even mentioned putting some blame on Odin for that. "Nor can I find it in my heart to hate Sigyn - even though she destroyed Blake. Or was it Lord Odin who did so when he played his dangerous game with the life and form of a mortal? Or was it I, who ne'er in all those years asked myself the questions which might have led me to that cave in time to save that life? We all must live with the guilt we share for Donald Blake's tragic end." Mighty Thor #483
this speech, given by the hero of the piece, is indicative of how the author wants us to feel about that situation. The same as when Steve says to Tony about Nick Fury having the same blood on his hands Loki does, that is how the author of the piece wants you to see it, because those words are coming from the hero, who is generally, the most truthful person there.