It can't possibly be wrong that Peter knew the cross was good for salvation since there could be no salvation without it.
Whether he knew it was good for salvation or not is irrelevant.
Again, the challenge to you was:
Demonstrate from scripture that Peter was preaching the cross as good news in Acts 2. You cannot, because it's not there.
It is, in fact, wrong that Peter taught that the cross was "good news."
“Therefore let all the house of Israel know assuredly that God has made this Jesus, whom you crucified, both Lord and Christ.” Now when they heard this,
they were cut to the heart, and said to Peter and the rest of the apostles, “Men and brethren, what shall we do?” Then Peter said to them, “Repent, and let every one of you be baptized in the name of Jesus Christ for the remission of sins; and you shall receive the gift of the Holy Spirit. For the promise is to you and to your children, and to all who are afar off, as many as the Lord our God will call.”
“Therefore let all the house of Israel know assuredly that God has made this Jesus, whom you crucified, both Lord and Christ.”
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You don't get "cut to the heart" when you hear that you've done something good
You don't repent of something that is supposed to be a good thing!
In other words, they were convicted of the bad thing they had done. And so when they asked what they should do, Peter said to repent and be baptized for the remission of their sins!
That's condemnation, not something to uplift!