What if doing no harm to one means deadly harm to another? Should I choose harm of lower level? Can I be a judge? Can I harm someone or something who is intending to do harm to someone?
There are no easy answers as there are no such thing as objective good.
For example should one kill a terrorist, who's aiming to bomb a plane? Should one kill innocent brother of killed terrorist, because innocent brother aims to take revenge by undertaking his late brother's plan?
If one has single free hand to grab one of two childs from a sinking boat, which child should be harmed?
Can I kill an animal in Antartics to feed myself, because no plants grow here?