You don't understand the female mind.
Or, for that matter, the human in-love mind.
She wanted Makoto to be happy, and for the looks of it, that was alongside Kotonoha.
Yet she couldn't resist her attraction towards Makoto and ended up kissing him. She knew it was a mistake and stepped back.
Then Makoto went after her and that was too much.
I don't blame Sekai at all. I feel sorry for her, for falling in love with such an ass.