Evidence for Peter in Rome. I don't know why people question that Peter went to Rome and was with Paul when they were martyred. Here is some non biblical evidence,
Peter tells his readers that he is writing from “Babylon” (1 Pet. 5:13), which was a first-century code word for the city of pagan Rome. Further, the Fathers are unanimous in declaring that he went to Rome and was martyred there under the pagan emperor Nero.
Ignatius of Antioch
“Not as Peter and Paul did, do I command you [Romans]. They were apostles, and I am a convict” (Letter to the Romans 4:3 [A.D. 110]).
Dionysius of Corinth
“You [Pope Soter] have also, by your very admonition, brought together the planting that was made by Peter and Paul at Rome and at Corinth; for both of them alike planted in our Corinth and taught us; and both alike, teaching similarly in Italy, suffered martyrdom at the same time” (Letter to Pope Soter [A.D. 170], in Eusebius, History of the Church 2:25:8).
Irenaeus
“Matthew also issued among the Hebrews a written Gospel in their own language, while Peter and Paul were evangelizing in Rome and laying the foundation of the Church” (Against Heresies, 3, 1:1 [A.D. 189]).
Gaius
“It is recorded that Paul was beheaded in Rome itself, and Peter, likewise, was crucified, during the reign [of the Emperor Nero].
Tertullian
“But if you are near Italy, you have Rome, where authority is at hand for us too. What a happy church that is, on which the apostles poured out their whole doctrine with their blood; where Peter had a passion like that of the Lord, where Paul was crowned with the death of John [the Baptist, by being beheaded]” (Demurrer Against the Heretics 36 [A.D. 200]).
Eusebius of Caesarea
“[In the second] year of the two hundredth and fifth Olympiad [A.D. 42]: The apostle Peter, after he has established the church in Antioch, is sent to Rome, where he remains as a bishop of that city, preaching the gospel for twenty-five years” (The Chronicle [A.D. 303]).
Peter of Alexandria
“Peter, the first chosen of the apostles, having been apprehended often and thrown into prison and treated with ignominy, at last was crucified in Rome” (Penance, canon 9 [A.D. 306]).
Optatus
“You cannot deny that you are aware that in the city of Rome the episcopal chair was given first to Peter; the chair in which Peter sat, the same who was head—that is why he is also called Cephas [‘Rock’]—of all the apostles; the one chair in which unity is maintained by all” (The Schism of the Donatists 2:2 [A.D. 367]).
Epiphanius of Salamis
“At Rome the first apostles and bishops were Peter and Paul, then Linus, then Cletus, then Clement, the contemporary of Peter and Paul” (Medicine Chest Against All Heresies 27:6 [A.D. 375])
Jerome
“Simon Peter, the son of John, from the village of Bethsaida in the province of Galilee, brother of Andrew the apostle, and himself chief of the apostles, after having been bishop of the church of Antioch and having preached to the Dispersion . . . pushed on to Rome in the second year of Claudius to overthrow Simon Magus, and held the sacerdotal chair there for twenty-five years until the last, that is the fourteenth, year of Nero.
Augustine
“If all men throughout the world were such as you most vainly accuse them of having been, what has the chair of the Roman church done to you, in which Peter sat, and in which Anastasius sits today?” (Against the Letters of Petilani 2:118 [A.D. 402]).