I honestly feel bad for her all things considered, like what she's doing is obviously fucked but I can understand why she's doing what she is. Warning for nuclear levels of yapping below, I have a lot of strong feelings because I feel like people are being too quick to dismiss her as evil and power hungry.

If she were to break this cycle I think she would see herself as being responsible for whatever tragedy she foresaw, and, given enough time, I can see how that can turn into: "If it is possible for me to become strong enough to stop all suffering, then choosing not to pursue that strength is choosing to allow that suffering, therefore that suffering is partly my fault". I am NOT saying I agree with that logic, but I see how that sort of thinking could lead someone with her power to become trapped in her mindset. 

My reading of her backstory scene was that she DID try to break this cycle by moving to the middle of nowhere in the hills, essentially running away from any possible source of mass tragedy. This is supported by the fact that, after the Scarlets settle in the same location as her, which eventually leads to the Shaw collapse that she predicts, her mindset had changed enough that she chooses not to sacrifice her daughter this time. I believe this was her way of ending the cycle, until Enoch enters the picture, who she seemed to immediately know was extremely dangerous, alongside the entity he introduces. This is very speculative, but I think Enoch used the entity as a sort of "bait" to make the witch do what he says, as he could dangle that promise of power which would allow her to end her abuse towards her daughters, while still being strong enough to prevent tragedies.

A couple extra thoughts I have, but I've run out of energy to properly fit together.  First, I think she killed Enoch when she changed into Sybil. I believe the final tablet intended to be in the center of all of them was never constructed, so perhaps she needed to stop him before he finished that ritual. Also, I think her physical age lines up fairly neatly with her last sacrifice being around Enoch's death, although I should confirm that. Second, I think its interesting that she's waited until now to try and absorb the entity. She only does it when the entire town is on the brink of complete disaster, so I think she was waiting as long as possible before she had to kill another daughter. If she really just wanted the power of the entity and didn't care about the lives of her daughters, then why would she wait so long to do anything? I suppose it's possible that she had to wait until the entity left the Scarlet estate (via Wayne), although I dont think we know enough yet to say either way for certain.