Warning! Spoilers ahead for Blue Exorcist chapter 135!
Previous flashbacks in Blue Exorcist have shown that Rin's mother Yuri died due to complications from childbirth, but Satan just told his son Rin something completely different in the present day that changes everything if true.
Before giving birth to Rin in chapter 108, Yuri's physicians realize that she's weakening and will likely not survive the childbearing process, but she of course is determined to protect them. Later during her contractions, experts become even more assured that Yuki will perish if she continues and, what's worse, she's eventually forced to deliver without medical help because of the curse that the pregnancy had put on her body. Then there's the added trauma of high-ranking exorcists trying to kill her newborn sons, which obviously didn't help her condition, so when Yuri and her sons' future adopted father Shiro Fujimoto later escape with Rin and Yukio into a snow-ridden landscape, it's just too much for her and she dies. In fact, chapter 117 confirms that Yuri was in more danger than Fujimoto who had just stabbed himself in the neck when Satan tried to possess him.
But in Blue Exorcist's latest installment, chapter 135 by mangaka Kazue Kato, Rin asks his father Satan during their pivotal fight why he was attacking humanity since he loved Yuri. And then Satan says it. First, the demon tries to remember who Rin could be referring to and when he finally figures out who Rin was talking about, Satan says that the ugliness and stink was so unbearable that he got rid of her. By got rid of her, does Satan mean that he killed Yuri?
At first, it doesn't seem likely, not just because Yuri likely died of complications from childbirth as noted earlier, but because Satan was truly obsessed with her. Upon meeting Yuri as a child, Satan killed her family so that no one else could have her, and that possessiveness just worsened the more he came to know her. It's also strongly implied that the infamous Blue Night began because Satan had become unstable when he felt Yuri was getting farther away from him. Satan once tried to possess Fujimoto, a man he hated more than anyone, because Yuri loved him and Satan wanted to become the man she loved. How could Satan kill someone he was so obsessed with? Could Satan just be lying to Rin to bait him? It's possible. But an earlier comment from one of three Grigori supreme advisors of the True Cross Order in Blue Exorcist chapter 117 could corroborate the theory that he could "get rid of her." After Yuri gave birth to her sons, the Grigori pleaded that Yuri sacrifice herself to sever Satan's attachment to her. Even though this could be interpreted as the Grigori saying Satan only wreaked havoc in the world because Yuri still lived in it, so if she were no longer in the world, there would be nothing to attract him there. That said, when Yuri dies, why does she appear in Gehenna? Did Satan form some connection to Yuri that enabled him to drag her down to from the world of the living into Gehenna? It's possible, and Satan's comment to Rin about getting rid of Yuri supports this theory.
Regardless of how she died, however, Yuri's story will always be a tragedy. But it could - and most likely already did - help empower Rin. The latest chapter of Blue Exorcist ends with Rin undergoing some type of awakening. Rin's anger at Satan for killing his beloved mother was undoubtedly one of the major factors that pushed him to the edge and unlock some new power. Hopefully, Blue Exorcist will allow Rin and his younger brother Yukio to avenge her.