Well, any answer to this question will rest on assumptions.
e.g. #1 "In order to feed oneself."
Assumptions:
1. The necessity of the physical survival of the Self.
2. That the continued existence of the Self is more important than the continued existence of the being which one seeks to transform into food.
e.g. #2 "In order to defend one's own life."
Assumptions:
1, 2 are almost identical to 1 and 2 above. (n.b.: I will not repeat assumptions that have already been mentioned)
3. There is a moral system which devalues the existence of the beings who do not abide by it.
e.g. #3 "Whenever that being is a source of annoyance or interferes with present or future comfort."
Assumptions:
4. My comfort is more important than the life of another.
5. A different type of being (e.g. a fly, a weed) is not as valuable as another type of being (e.g. a human, cattle).
I could give more examples, but it is worth noting that in just about any example I could give, this assumption stands before all others:
=> The being that is being harmed or killed by Self is separate from Self.
OR
=> The Self is not worthy of life.
Essentially, the point of this post is to note that, whatever your reasons are, you must confront the assumptions whereupon they rest. Do they contradict what you believe? How is it justified to make the assumptions you are making?
Agapooka