Protagonist decides that going out with a guy you didn't like was better than confessing and then ultimately decides that killing the girl she loved and burying her in a shallow unmarked grave was better than confessing
The first part is a common trope and is perfectly logical. The chances of her love both being gay and returning her feelings are very low, so instead of risking rejection and losing her completely, she does what she can to maintain the status quo.
The second part... well, yanderes gonna yandere. "If I can't have her, nobody else can"