The thing is, minimization of suffering is not the only thing to consider in ethics, it is a very complicated matter. For example freedom, the individual's right to decide themselves, is often considered at least on par with it; deciding to kill something, because you think it is best for them, is not often seen as very ethical.
Personally I think there just is no single solid ethical Truth, that decides what the Good is (since ethics are human things, and humans are far from infallible or all-knowing), it is more of an ongoing discussion, about what that means. But we should still strive towards that Good, even if we are not quite sure what it is, or change our minds about what it is. In fact, a person thinking they are absolutely sure something is Right, is a sign, that they should not slam that door shut. Only a sith deals in absolutes, essentially lmao.