I got the impression from the last page that Natsuko knows (thinks?) her friend isn't in love with her so all she can do is keep being cruel to her to keep seeing her cute desperate face.
Actually, it sounds like a brutally honest assessment to me. (Unusually insightful, given the age.)
"I don't need any friends. As long as you're with me, I don't need anyone else," is not what I'd call a healthy attitude. Micchan is someone who consistently excels academically, but needs Natsuko's affirmation to see her own self-worth. That borders more on dependency than love. I give Natsuko points for recognizing that a codependent relationship may not be a direction she wants to go; of course, given the deficit of points from her cruelty, that still leaves her in the red in my book.