In "Things Fall Apart," why did Igbo who killed a Christian have to flee from the clan?Why are the Igbo Christians upset about admitting the osu?

The osu are the outcasts, and the new Christian converts are horrified when Mr. Kiaga allows them to join the church. According to tribal tradition, if one is osu, all is forbidden. In fact an osu cannot attend assembly, take any of the titles, and when he died, he would be buried in the Evil Forest(Chapter 18). The osu are clearly identifiable in the clan as they are also not allowed a razor, so their hair is long and tangled. Kiaga stands firm in his belief that osu can join the church. One convert goes back to the clan. The other converts take strength in Kiaga's firm belief and the new Christian religion; the osu shave and join the church becoming the strongest members of all. Of course this makes perfect sense; once they were outcasts and now they are accepted.