Medusa the Witch- The One Who Possesses a Great Evil Soul?
When last we met Maka and a severely-injured Soul faced down against Crona, a meister whose weapon is his blood and whose weapon is a massive overbearing bully driving him to devour human souls. Dr. Stein and Maka’s father arrive just in time because it turns out the Witch Medusa had infused Crona’s weapon into his soul in an attempt to make a demon sword.
Maka learns that keeping meisters and weapons from being corrupted like this is why the Academy was founded, and also that Demon Swords are powerful weapons that are created when a weapon overwhelms the soul of the meister, turning them into a savage magical monster. She also sees a powerful witch soul for the first time and begins to learn how witches (and everyone else) can manipulate their souls through magic (much like Dr. Stein has manipulated his body.)
So now (Foreshadowing!!) Maka *should* know that evil (and danger) can be anywhere, even disguised as a friend. DumdumDUM!