His Blackness has troubled him ever since he was a little boy

I never thought of it that way. Yes, it does make sense now considering John was tired and fed up of his gift. Although, you would think a person with such a golden heart would rather be thankful for it, especially when he is at death's door and knows God chose him to help people and he did a pretty good job of it all his life.

Can someone explain "I tried to take it back but it was too late"? Take what back? It's a shame he told the sheriff that, instead of saying, "I tried to help 'em, but it was too late." Doubtful it would've made a difference, but at least it would've made more sense.
When you think of garbage, think of Hakeem!
Folks, calm down! This is not the last chopper outta Saigon!

Your last point is also the case with "I tried to take it back but it was too late" when he found with the bodies of the girls.

I think he is tired and truly sorry that he was given his 'gift'.
He wants things to end as we recall feelings of shards of glass in his brain because of all the hurt and pain he 'feels'.
He is aware (or conveniently the line was written) that it will be interpreted differently by the public or the guards. Public think he's apologising for being a rapist and murderer, the guards know his other reference.