Than why arjun fight and kill the enemis even though they are his relatives?

Because Lord Shree Krushna told him that heaven or hell should not deter a person from doing his duty i.e. karma. Arjuna's duty was to protect the dharma i.e. law and to punish theadharmis i.e. lawbreakers when they came to fight him he had to fight and vanish them. When a person does adharma, that person has crossed the limits of tolerance of dharma and needs to be annihilated and so the person who commits such acts is an enemy of mankind and needs to be stopped, so he had to kill the enemies even though they are his relatives.