Some Christian apologists (William Lane Craig, Is There Historical Evidence for the Resurrection of Christ?; Mike Licona, The Case for the Resurrection of Jesus) argue that the resurrection of Jesus must have been a real event because otherwise it would be impossible to explain the availability of the apostles to be martyred for witness: if Peter had not been convinced that Jesus was truly dead and truly risen, he would not had let himself be killed in his name, but he would have confessed the deception and got saved.
On this topic, some preliminary considerations should be discussed: for example, we are not sure that Peter was indeed martyred, nor we know with certainty that the deny the resurrection of Jesus would have saved his life, nor can we rule out that he had no particular reason to prefer death the unveiling of lies (think of Ron Hubbard and Scientology). But for this discussion, we will assume that Peter would deny Jesus without further consequences, but he chose to die to bear witness to his resurrection.
It should also be noted that the argument presented is valid only if the apostles (Peter) knew that Jesus had died and had risen again: it is important to emphasize that their mere belief in the resurrection is not sufficient evidence to demonstrate its actual historicity, as it is possible that they were wrong. In other words, with respect to the relationship between the resurrection and the apostles, there are three possible scenarios: the apostles knew that Jesus had risen, the apostles believed that Jesus had risen (for example, through a re-reading of the biblical allegories to understand the reasons for the death of their master, an event incompatible with their faith), or the apostles knew that Jesus had not been resurrected (and lied about his resurrection). Of these three scenarios, the only favourable to the apologists is the first one, in which the apostles are witness of the resurrection (as described in the Gospels, for example) and not merely convinced of its occurrence.
Well, the apologetic defence of the resurrection is just disowned by the existence of the Christian martyrs! Of all the Christian martyrs, in fact, the vast majority were killed for their faith, because they were convinced of the resurrection without being witnessed; just a minority of them (Peter, besides the "other" apostle and the women, if they were actually martyred) died because they were convinced by the facts.
Of course, it is possible that Peter was an eyewitness of the facts and died to bear witness of the resurrection, while the other (tens of) thousands of dead for their faith have taken this step without being witnesses of that event; this does not affect the possible truth of the resurrection. But what this reasoning proves is that the argument of the martyrdom of the apostles is not evidence for the truth of the resurrection as, if anything, of the fact that the apostles were convinced that it had taken place without being really been witnesses.
The picture is The martyrdom of Saint Peter, by Michelangelo Merisi da Caravaggio, through Wikimedia Commons.