I found this question very interesting because it shows some phenomenon:
The Buddha said:
Refrain from killing any sentient being.
But the thing is, you can kill the "sentient being" just to sustain life of another individual (food), or even save another "sentient being" (tapeworm and another parasites).
Even with mosquitos, you can kill them to save somebody from malaria (in extreme case), or just to make the population smaller, and less annoying to the human kind.
The key to understand what Buddha meant is probably appropriate interpretation of word :
Did the Buddha mean one should kill only when necessary?
Where should buddhists draw a line?
Should he/she kill mosquitoes sitting on his/her body?