That all depends on two things for me:
1)What this other being is. Are we talking about a person or a bacterium ( or where between), and from that how would that being's ceasing to exist effect its environment? I would need more details before weighing in on a solid answer.
2)What would be our reason for destroying the plant? What would define unnecessary? The plant could be sustaining that same being (a bacterium in this case) that in existing kills other things humans depend on (a crop that humans depend on). Nothing exists in a vacuum, so more details would be needed in this respect as well.
EDIT:
Reading my post over, it looked really wishy-washy, so as not to make it a
complete cop-out, I think it would be wrong to unnecessarily kill a berry plant (that I will define as a being in and of itself, if going by the definition in the OP and just to complicate things
) if doing so would stop a being from existing, but this is with just the two variables of necessity and dependence being known. As I said before, I would need to know more before coming to a strong answer.