Explanation:

Vermin are nuisance animals that destroy crops or spread diseases and hence their populations need to be regulated.
Generally the State chief wildlife warden assesses the situation and presents a report to the Union ministry of environment and wildlife. The ministry on the basis of that report classifies an animal as vermin and places in the Schedule V of Wildlife protection act 1972. This allows for legal culling or hunting of these animals.
Recently the states of Himachal, Uttarakhand, Bihar etc. declared  Nilgai, Wild Pigs and Rhesus Macaque as vermin due to the damages to the crops they cause and thereby allowing for their legal hunting and culling.
This culling should not be seen as unethical as this is essential for long term sustainability and overall health of an ecosystem.