why do people need to kill mosquito larva during prevalence of malaria ?

This is a way of controlling the spread of malaria. Killing the mosquito larvae will reduce the population of mosquitoes, which are the vectors of malaria. When a carrier mosquito bites a human, the parasite (genus Plasmodium), which causes malaria, is transmitted to humans with mosquito saliva. So killing the mosquito larvae which are usually laid in open water controls the population of mosquitoes and thereby spread of malaria.