Islinda had no idea what she had been thinking when she raised that knife at Aldric except that she had been blinded with so much rage and misery that she wanted revenge. She wanted to hurt Aldric as much as he hurt her. He needed to feel how much pain he caused her. How could someone be so empty of human emotion? Except he wasn't human. He was Fae.
Even with the desperation to harm him, Islinda knew she wouldn't get within inches to hurt him. She has fought Aldric enough to know that he was stronger than the regular human and would always see her coming. Perhaps, she banked on that expectation and though she would lose to him, the fae would know that he hurt her deeply. Not just her heart, but her soul.
Hence Islinda did not expect that he would not make a move until the last minute and put his hand in the way instead. The wail died in her throat when she saw the knife embedded into his flesh and then blood trickled along.