Lisa IS "the Demon Lord!"
She's the one who gave Sain that Eye to begin with and has been ENCOURAGING his evil behavior ever since! Even the "Dumping/Exile" back in Ch1 was by her encouragement! And Sain doesn't have a single clue! So if you think he has some innocence "don't think it" all she did was open the NTR scenario for him, he took it hook line and center so he is vile
Oh and if you have any thoughts about Lisa caring about MC, don't, she used him since day 1! "She was the one who killed his parents" by the way. For the "EVULZ" all while "seducing and dating him" simply because she knew it would get Sain's attention! She COULD kill Sain, but then she'd lose track of the Hero Title!
Her snag as she is unaware of it, was MC! She assumed he returned to the village in disgrace....or dead in a ditch, like with Nei and Soara. She stopped thinking about them once they were "thrown away" while aware SOMEONE saved the city from her general, she still believes Nei is dead, and Soara is "somewhere" as some POS "Sex doll!" Toru ruined EVERYTHING! And well she's just as unhappy to find out the Brigade that kept getting in the way was run by Toru as Sain will when they meet again. She never hid her evil manipulations because Soara was too brainwashed to do anything, but by having her sold off, it assured Toru found her and is now more weary of her, especially when she gloats about everything she did and how she had FUN faking it since DAY 1 and his "parents" thing! Even betraying him and making him think HE was the one in the wrong was FUN to her, cause Bitch is like "Junko" from "Dangan Ronpa" she LOVES bringing Despair on others! But this also assures any LINGERING love Toru has for her DIES which is good for Kaede as her "lingering prescene" in his heart is the reason she and Toru can't go 100%. After the full truth. Yeah Toru is full on "bitch needs to die!"
She was also the one to SUGGEST everything Sain did that was bad, from setting Toru up with that scenario in Ch1, to abandoning Nei, to even SELLING Soara after bringing in a NEW Priestess to replace her! By the way the "Replacement" is a "MALE DEMON in disguise as a female" because you know Sain is a perverted POS, and it's implied they had "butt sex" when the reveal happens and the demon pats his ass!
Toru will only full confirm what Soara said when he throws the water on her...... and it doesn't work! Revealing she was never brainwashed and if I remember, it forces her disguise off! Revealing her TRUE Form and "levels" to Toru and Sain! Lets just say "She's double Toru's current Level! And Sain is NO WHERE near Toru's Level, so yeah, if she felt like it, she could and WOULD have killed the part and him specifically AT ANYTIME! And remember, the charm "only works on those BELOW Sain's current level! It just confirms to Toru she was ALWAYS a fake ever since they met, though Sain goes into denial as this means he "never stood a chance against Toru" because Lisa GAVE herself to him willingly....because she was 100% using him and he never had her heart....because she never had one for anyone BUT herself to begin with! And yes she IS the final villain of the story.