It's extreme behavior, yes. But it isn't extremism. "Extremism" implies extremist ideals or views. Her murdering Tamora is more of an emotional reaction mixed with her strange way of rationalizing things.
In her mind, her only goal in life was to set Tamora free. Tamora's captor was the Yeerk Empire and, when she became voluntary, the only way to free her of the Empire was to eliminate her.
Also, hi Shock! And yeah, Terenia isn't manic strictly speaking. She's a bit schizophrenic, and suffers from slight DID...which you'll have with three different species instincts yelling in your head about how to react on top of your own mind and possibly a hosts. But mostly she's just paranoid and high strung and suffering from PTSD.
