First off, let me say I oppose police being armed with deadly force as routine. And am absolutely opposed to the death penalty, without exception.
However, your position that one has to be anti-arming if cops to have sufficient morality to oppose the death penalty doesn't make sense. Police are armed for more reasons than to administer justice. Indeed, the only justified use of deadly order ever, in my opinion, is to prevent immediate death of another. That has nothing to do with justice or the death penalty. It has to do with exigency. Police use of deadly force in exigenr circumstances is not at all similar to execution after years on death row.