She didn't stop. Shepard had never once in her life given up on anything. Through death after death, life after life left in her wake, she moved onward, and upward. You could either stand by her side or be ground under her heel-- or, in Karkat's case especially, carried on her back.

Deafened by the bedlam of the Cornucopia and the noise of the Gamemaker's retaliation for the inborn powers they themselves had permitted the Tributes to use, Shepard was unaware of Karkat's mumbled protestations. She did not feel the way his breath slowed, and it would be some time yet before she realized the futility of her heroism. Karkat Vantas was beyond her help.