Acts 3:11-16. After Peter heals the lame man, many people run to see and are astounded. When Peter sees all the people, he asks them why they wonder at this. He says it is not by his power or piety that this man was healed. He reminds them that God sent and glorified his Son and servant Jesus Christ who they denied before Pilate, and who they condemned to death by asking for a murderer to be released in His place. They had the Author of life killed, but God raised Him from the dead. It is through the name of Jesus, and through faith in Him, that this man was healed, Peter says.

I am assuming that this is a different group of people than the ones Peter addressed at the first Pentecost, reminding them that they had killed God, but there probably were a few of the same people, and Peter reminds them again. This is a point it is necessary to drive home, because Jesus Christ must be known to be the source of this amazing power now flowing through the apostles.

People have to realize what they have done, especially when they have sinned greatly. They have to repent. Only then will the grace of God be able to flow into them - when they have made room for it by humbling themselves through repentance and penance. There is no greater sin than killing God, so Peter wants everyone to realize what they have done so they can repent and receive grace. His words apply to all of us, as we "kill God" each time we sin, and Peter likewise wants us to repent and make room for the grace.

If we don't see Jesus,.and acknowledge what we've done to Him, we will fail to see the connection between His followers' and God's power. In order for Christ's message to spread, for it truly to  take root in our hearts, we all have to be reminded of who Christ was, and what we did to Him. So Peter is not just rubbing it in, he's trying to help us see the connection, for our own good.