If the only thing a man can save are the people on the side he's on, then why does Rin save Shinji and why does Archer save Shirou?
Kerry's methods and philosophy were fundamentally wrong. He was never a hero, and he would never save anyone. By adopting evil to fight other evil, he ensured his own defeat. In the end, he was broken and unfulfilled.
I would argue that his philosophy was fundamentally right, but he stuck too those fundamentals too much, and never made exceptions or deviated from his own arbitrarily strict rules. Being able to save the people around you, sacrificing few to save many, being able to only save people on your side, on paper, they sound good, but in practice, like in the boat situation, it's not always the best thing to do