Irresistible

"Why did you do this? I treated you well, loved you and never wished you
any harm. Why did you betray me?"
The sweet-faced girl looked at her with eyes filled with tears, sadness and
disappointment, and it made her smile. Tears welled up in her eyes, but she
didn't know what kind of tears they were.
"Because..."
"Why, what?"
"Because..."
"Tell me."
"Don't know!"
My forehead hit the keyboard after typing 'I don't know!!', causing a letter
to stretch across the monitor. That was not the character's feeling; it was
mine. Although I was supposed to write the climax of a well-constructed
story, I was suffering from writer's block because I couldn't find any reason
for the antagonist to hurt FL.
In this story, the antagonist, who had always received love, care and
everything good in the world from FL, stabbed her in the back while trying
to steal from ML, when she caught him kissing her. Of course, ML had no
idea about this.