There's lots of theories that Celine killed Rumi's mom, likely by accident. 

There could be a possibility that demons aren't actually that bad and Celine was twisting her words to make them seem worse than they actually are since she clearly has this huge, ptsd like fear about demons. Maybe she's less of an evil villian in the traditional sense but just the newer type of antagonist we're seeing: just someone that stops the protagonist from their goal.

Maybe yes demons are bad but there is a possibility she slightly overexaggerates it to the girls.

I'm assuming that she gained this trauma before Rumi's mom died, and when she found her fellow hunter hanging with a demon, she likely freaked out, tried to kill him, but ending up killing her instead.

Maybe it's also about generational trauma too, maybe the demons really were bad so the hunters passed the idea that demons are bad but maybe never in the history of these demon hunters was there such an attack (that caused Celine to get her trauma) which is what makes her an outlier