Terezi Pyrope Vriska sees her former team mate Terezi as her rival, but also tries to stay friends with her. She was shocked when Terezi actually killed her, because she thought she would not be able to go through with it.

Who killed Aradia?

After summoning ghost on Vriska Serket for blinding Terezi Pyrope, Vriska got revenge by summoning a mindless Sollux Captor to kill her, which is why Aradia is a ghost when introduced. After having her soul be built into a robot by Equius Zahhak, she managed to get revenge on Vriska by beating her to death.

Is Feferi dead?

She is DEAD. He is not going to take it well when he wakes up. You hope he just stays asleep for a while.