Jothar- If your Rav says its necessary then you follow what he says. Only Hashem takes into account how punishment will affect others, we (Bais Din) do not.
In Shemos when Moshe killed the Mitzri that was hitting a Yid, the pasuk says ???????? ???? ????? ???????? ???? ???? ????? which we darshan to mean he saw that no good would come out of him or his descendants. The question is raised that why did Moshe do that, if the Mitzri deserved to die we dont look at anything else? An answer given is that since he used the ?? ?????? to kill it was like ???? ???? ???? which does make other considerations.