I've got mixed feelings about the story so far. The art is cool and dynamic, but Sif is someone I'm not entirely rooting for right now. She's got a berserker spell, but she's acting like a jerk, and I'm still not entirely sure of what's going on. Get Berserker spell, and then what? Who is she going to fight? What's she going to do?
I'm looking forward to seeing where this is going though. I'm rooting for her, if only because she shouldn't wind up being the supreme jerk of all the Marvel heroes.
Its just sibling stuff.
''How do you find someone who has spent a lifetime covering his tracks? For some, he was a guardian angel. To others, a ghost who never quite fit in''
Maybe I'm in the minority but I thought things didn't get interesting until the second issue. Sif's normal personality seemed pretty vanilla and boring before she got corrupted.
As for her state of mind, you're supposed to be concerned for her but I'm pretty sure the scene in question that the OP posted was not to be taken seriously. Seemed like really really dark humor, that's still supposed to have you raise your eyebrows but not be taken at face value.
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She is being driven crazy by the berserker spell that she is using for added power. Also it was mentioned in the first issue and is something most have sort of picked up due to their lack of direct interaction, Sif and Heimdall rarely have acted like brother and sister enough. In fact before 2011 I (and I am sure a few others) didn't even know they were related.
While I was initially worried about how it would seem to newer readers, having read the second issue a few times enjoying the art and story, I honestly don't care how they feel the story is good. Not to mention it is a different take to get a character used to the lead by having them literally battle outside impulses and get to the truth of the character.