I think it's possibly a combination of emotions that caused them to say "we forgive". Perhaps they had just finished listening to a sermon that inspired them to say what they did. Perhaps they were saying what people (their Christian peers) wanted them to say publicly. Perhaps there is a mix of genuineness as well as a desire to play up to the media. There may still be repressed emotions of hatred, disgust and a vengeful rage that haven't surfaced yet. Some people react slowly when in shock. The anger comes later.
I do think that as the media fades down, and as time goes by, that forgiveness will fester into anger and righteous indignation and the burden of loss will cause them to wish Roof gets what he deserves.