I knew she was evil, but not "literally poison my brother" evil. Dio Brando levels of evil.
I'm feeling like a lot of the bad or "evil" characters so far were deeply, deeply hurt by others, became twisted, and passed it on. Ryo who was cruel to Yuzu at first due to her grudge, Tamako, Tamahiko's half-brother, even the father.
Some of them broke the cycle.
But Tamayo is a psychopath. The way she tortured insects and her little sister when she was a kid was a tipoff, but poisoning her injured brother is different than just wishing he had died.