"But God raised him from the dead, freeing him from the agony of death, because it was impossible for death to keep its hold on him." NIV translation
Have you ever had a problem, done something about it, thought that everything was all better and then found out that your solution had only made the problem worse? I know that I have been there and done that.
That is basically what Peter is telling this crowd of Jews now. In the last verse, he explained that they were to blame (and in fact we all are) for Jesus going to the cross. Now, he adds that the solution they came up with (the crucifixion of Jesus) did not work because "God raised him from the dead". That's really got to get their attention.
Peter tells the crowd that Jesus was freed from "the agony of death". Do we really understand what that means? My friends, many people take comfort in the false hope that, after they die, life is just over. We see hear that, even for Jesus when he died, life was just not over but he was gripped in "the agony of death". There may be some really terrible physical things that occur when a non-Christian dies but can you imagine the agony that Jesus felt? He was separated, for the first time, from his father and took the punishment for our sins. Separation from God, my friends, that is agony.