What is the ruling on killing insects inside a wardrobe during the daytime in Ramadan, and does that affect the reward of the fasting person?

Sins diminish the reward of fasting. Therefore, killing insects that the Shariah has prohibited from killing, such as bees, while fasting, and knowing of the prohibition, is considered a sin that may reduce the reward. As for harmful insects or those for which no prohibition against killing has been reported, there is no harm in killing them.