@Stevie333 Er, no, the drugs are lethal too. The drugs kill the target without leaving obvious evidence (like a gunshot wound). In chapter 10 they had to get rid of the body because there's a gaping hole where the brain was. Kanako initially wanted to kill him off by lethal injection.
@Amazing_Retard Kanako murdered her for having a bad personality. That is, she refused to acknowledge her fault or even return the stolen book after being caught red-handed. The joke in this manga is that, "The people who do [x] should just die," fulfillment fantasy. Kanako's cleaning up annoying people from the world one murder at a time.