For example, if someone had shot Hitler knowing that he had ordered the Holocaust, and then was shot himself, that would be a good cause based on facts established by more than one witness, ie that Hitler did indeed order the Holocaust. However, not everybody knew that Jews were being killed in Germany. Only some people knew. Likewise only some people saw Jesus alive again, but enough to make it (for them) established fact.
The reason some of the first Christians died is because they were telling people that Jesus could give them eternal life, because he had risen from the dead. This angered religious leaders because it put them out of business. They were killed because of jealousy.
But the point is, you need a combination of testimony from multiple witnesses to validate a cause as well as a willingness to die for it, to show that it is true. We have multiple written reports of Jesus being alive after his death, and we know that the Christians were willing to die so that their message got out, and also that their message was for the good of the recipients.