Am I the only one wondering if Regina could always posses people then why didn't she use that to kill Snow all along??
probably because possessing a body exposed her to the same dangers that body faces, i.e., if Snow were to die with Regina possessing her, Regina would die as well? Also,
I don't think it was a possession, I think it was just channeling her voice through Mary Margaret.
... and I would agree. We don't know if Regina actually shifted her consciousness into Mary Margaret, or used her as conduit/relay. When Regina creates/releases the smoke, we see her concentrating, but the camera shifts before we see if she goes into a trance or something. Lastly, she said she had "to make this fast", and whether that is a condition of the spell or just to avoid discovery, we can't say.