Why is it important in Islam to convert other people?
Well it isn’t. A Muslim job is to only to convey the message of Islam. Discussed here [first 1 min];
http://video.aol.co.uk/video-detail/gary-miller-nature-of-belief-part-88/1698501648
Why is a non Muslim referred to as a kaafir?
Will write about it later, Insha’Allah