I mean, she is a healer and highly loyal to her master and friends. She has seen first hand the atrocities committed by the separatist forces. Even if she did not like the Jedi Order's actions, how come she goes Dark?
It's hard to answer exactly as you don't provide a clear frame of reference in your question, but Barriss Offee was ultimately convicted of several crimes and sentenced to jail by the Republic courts. The crimes included:
- orchestrating the bombing of the Jedi Temple (resulting in the deaths of Jedi, civilians, and Republic troopers.
- the murder of Letta Turmond.
- coordinating and working with an enemy of the Republic: Asajj Ventress.
- Attempting to frame Ahsoka Tano for all of the above.
Upon being found guilty, Offee was remanded into Republic custody and imprisoned (I don't think it ever states where). We do not hear of her again in any canon capacity to my knowledge.