Opinion about Takano Mio
Vicious murderer or some what of a victim of circumstances that's gets rehabilitated at the end?
Don't know my self, in the story her appearance and behavior is quite contradictory.
Have you watched Kai yet? If not then SPOILERS!!!!!!!!
Remember the men in the van that tried to run Keiichi over in the first arc of season 1? Yeah, those men were hired by Takano Miyo to help her fake her death and kill anyone that gets in her way of becoming immortal (like Rika). She killed Tometake by giving him a special drug dose which would enter his blood stream and leave after he's dead. Takano was a despicable character, that's why the ending of "Festival Music" arc was so fitting.
If you haven't watched Kai or Rei then you probably should, more of your questions will get answered. OH, and to answer your question now, SPOILERS! She tries to kill Keiichi, Rika, Rena, Mion, Satoko and Hanyuu towards the end of "Festival Music" arc, however Hanyuu steps in by basically becoming Oyashiro-sama and diverting Takano's bullet so that she'd miss. She however, does NOT become rehabilitated. If you've seen the worthless OVA Kira, the first episode shows that she will never change her ways in each Hinamizawa as because she infects Keiichi with New Hinamizawa syndrome in the "Penalty Loving" arc.
Sorry for the abundance of spoilers, but hey, that's to answer YOUR QUESTION! WATCH THE ENTIRE SERIES FOR MORE ANSWERS!
Interesting points, I watched the first season and then the second named "kai".
At the end of kai Mio ends up berated by her team and ends up herself as a victim. Both the heroes(Rena, Keichi and others) and Tomitake pities her.
In the final shoots a young Mio is confronted by Frederica/Rica and one gets the impression that it led Mio to live a normal life with her parents and she didn't do any evil stuff in the future.