Stuff I don't get

Why wasn't Marie's shirt covered in blood from killing the mother? The blood sprayed everywhere. The gas station clerk should have noticed that, but in the film he only seems to notice a small amount of blood on the killers hand. Plus in the surveillance video I don't think Marie's shirt is covered in blood.

Was Marie really Alex's friend or did she imagine the whole thing about them being friends and driving to Alex's parents home together? This part still has me drawn. Either she really was her friend and just went on a killing spree one night or she was an obsessed killer who decided to show up one night. Supposedly a photo of Marie is visible above the fireplace, and when Marie unties Alex Alex doesn't immediately stab her (which if there is a pre-existing relationship there might be some hesitation on Alex's part to stab Marie at first).