It's not a traditional "whodunnit" because we find out who does it exactly when the murder takes place, but with all the speculation leading up to it, it could currently be classed as a "whodunnit" as everyone is wondering who would kill Tina, why, and how. Also, the characters in the show won't know who did it (with the exception of Tina herself and the killer), so in-show it is a "whodunnit".