But surely it is good to kill sometimes. To kill disease-spreading insects, for example, or someone who is going to kill you?

 

It might be good for you. But what about that thing or that person? They wish to live, just as you do. When you decide to kill a disease-spreading insect, your intention is perhaps a mixture of self-concern (good) and revulsion (bad). The act will benefit yourself (good) but obviously it will not benefit that creature (bad). So at times it may be necessary to kill but it is never totally good.