The old lady is the devil.
The devil is in the elevator to claim the souls of people who have lead a life of sin. While the people are trapped in the elevator, a cop, Detective Bowden, who lost his family to a hit and run and is now a recovering alcoholic is trying to figure out how to get them out. Everyone is killed in gruesome ways except for Tony, who the devil reveals himself to. It turns out Tony was the one who killed Detective Bowden's family. Over the elevator two way radio system, Tony repents his sin and is genuinely sorry for leaving the scene of the accident. The devil leaves him, sparing his soul.
During the closing frames, Detective Bowden says he's been waiting to meet his family's killer for a long time, but can only forgive him now.