The most difficult relationship to maintain is that of a marriage. Given hardships or persecution, that difficulty becomes even more so. But what if your spouse isn't even a Christian yet?
It is one thing to simply put away bitterness towards our wrongdoers and it is yet another to honor them! Jesus' crucifixion is a perfect illustration of the later.
In the face of persecution for their faith, Peter encouraged the Christians who were scattered to do one main thing; put away any bitterness towards those persecuting them.