Why was Saint Peter executed?

Answer 1

Nero had little use for Christians but there was little he could do since they were not causing any problems in Rome. Some say that Nero wanted badly to build a new Rome in his own style and he had the city intentionally set ablaze in 64 AD and then blamed it on the Christians. He used that as an excuse to arrest and execute the Christians, especially the leaders of the new religion.

Answer 2

Acts of the Apostles mentions the imprisonment and miraculous release of Peter, but does not say anything about his subsequent death, even though the book was written around the end of the century, long after Peter would have died. A second-century tradition was that he was beheaded, while a later tradition was that Peter was crucified upside down. In fact, the story in Acts is dubious at best, and the later traditions have no evidence to support them.

In summary, we have a biblical author who knew nothing of the manner of Peter's death, although he claims to have known more about Peter than any other source, and two unreliable and contradictory traditions. Put simply, we do not know whether Peter was even executed. We can not say why.

What we know about the historical Saint Peter can be read here