Vanity

Yes exactly! I think the ''It's watching'' hints even further to a big brother

Sorry to burst your joy or bubble but the killer is actually named in the teaser Nii is japanese for 2 and 3 in japanese is San So Nii san / Niisan For all who didnt waste forever watching anime NiiSan Means big brother and you hear the heavy footsteps, its going to be a japanese male with some kind of big brother role ^^