Jill thing is a much heavier betrayal then Sage could ever be. Sage cheated on Chad, Chad cheated on Sage, so in essence they commited the same shit MC did, and if you have a love interest with Sage but go through with another char, she will literally say that she understands the situation and that she really does not have hard feelings at all. Jill is basically a virgin who had her first whole lovelife encapsulated into MC, only for it to be absolutely betrayed by her best friend whos basically her older sister, her only confidante and her best friend, fucking MC, the guy whos literally "The one" for Jill. And without Tybalt, shes literally alone, no one is there for her, and that brings people into dark places.
Imagine your best friend, fucking with your love interest, the one who you thought was literally it, the one who you knew you wanted to marry someday and have kids, and live ever after, only for that guy to fuck the only best friend you ever knew, and for them to do it right by your face, and keep in mind, she has no one to "Vent" out to, that would fuck me up.