Question

who killed gandhi ji and why

Answer 1

Nathuram Godse killed Gandhiji and shot three times in his chest. He killed him because of Gandhiji's secular ideology. 

Answer 2

Nathuram Vinayak Godse was killed the mahatma gandhi  due to Gandhiji supported the idea of a separate State for Muslims. In a sense he was responsible for the creation of Pakistan.
 In spite of the Pakistani aggression in Kashmir, Gandhiji fasted to compel the government of India to release an amount of Rs. 55 crores due to Pakistan.
 The belligerence of Muslims was a result of Gandhiji's policy of appeasement.