They were children of war, both of them. perhaps they hadn't seen true war itself until they were nearly eighteen, but that didn't change the fact that they were born of it, that they suffered because of it, that they were nothing but pained remnants of what once was.
Of the two, Karin had come out better. Sasuke could have done well, but Karin had been the one to survive, mentally. Sasuke had lost his mind, lost sight of what was important, and left behind a village that could have given him everything that was important to him. He'd driven himself mad with grief and hatred, and had to be pulled back from the edge, pulled back from over the edge, time and again before it stuck.
Karin had been different. Her village had been destroyed, and she had pulled herself together and gone to Orochimaru, because she had nowhere else to go, and he gave her purpose. Karin had gone because she had the potential and nowhere to train it. Karin had gone because the one person she had left in any way, Sasuke, had gone there as well.
She'd be damned if she let go of the one person she sort of had left.
And so she followed him. She came when he asked, helped when she was needed, and did her best to try to bring him back to what he needed to be. Her best wasn't enough (no one's was, no one but Itachi), but she tried. She tried so hard to protect him from the biggest threat he faced:
His grief and madness.