What does Obierika believe about Okonkwo's decision to help carry out Ikemefuna's death? What might this foreshadow?

He thinks it is the beginning of the end for Okonkwo. Once he sheds Ikemefuna's blood, he will never be the same. Things go downhill for Okonkwo after this.

Obierika also feels that Okonkwo is violating tribal law by performing this on his own son. He feels that Okonkwo’s actions seriously damage both his relationship with Nwoye and Nwoye’s allegiance to Igbo society.