If you run into the girl you met on the side of the road again in some random bar this wouldn't be a coincidence. Also it smacks of evil. Shadows are usually harmless but not this time when they're killer shadows being controlled with some black mojo. Destroying the altar would be a good idea.
Meg sets a trap drawing the boys in so they will call Dad. If they just left the Winchesters alone all those years again they wouldn't have to worry about John Winchester hunting so they have no one to blame but themselves. Meg takes a swan dive out the window and Dad shows up only to have the shadows attack again. Carry around a flare gun so you can make with the skedaddle. Yummy even drenched in blood. Oops did I just say that out loud or on the screen?