Lots of people die for beliefs that both you and I consider false, all the time.
Or do you think the Saudis who flew passenger planes into the WTC towers are now enjoying the attention of houris in heaven?
Do you think Joseph Smith went to his death for the Truth?
What about the followers of Mani, who kept loudly proclaiming that their founder was alive (Mani chai = Mani lives) even after the Persian king displayed his corpse and brought death to anyone who openly declared allegiance to the faith?
Even ideologies pretty much everyone considers (rightfully) despicable can produce uncompromising loyalty unto death: thousands upon thousands of young Germans died for the sake of the National Socialist world view, and even those who perpetuated it embraced it so wholeheartedly that they'd rather die (and kill their loved ones) than forsake it. When Goebbels and his wife realized that the war was lost, they wrote a letter declaring that they would not wish to live (or have their children live) in a world devoid of the ideals they held sacred, then proceeded to kill both their six children and themselves.
Her letter to her adult son (then in Canada) read as follows:
"[...]Unsere herrliche Idee geht zu Grunde, und mit ihr alles was ich Schönes, Bewundernswertes, Edles und Gutes in meinem Leben gekannt habe. Die Welt, die nach dem Führer und dem Nationalsozialismus kommt ist nicht mehr wert darin zu leben und deshalb habe ich auch die Kinder hierher mitgenommen. Sie sind zu schade für das nach uns kommende Leben und ein Gnädiger Gott wird mich verstehen, wenn ich selbst ihnen die Erlösung geben werde. [...] Wir haben nur noch ein Ziel: Treue bis in den Tod dem Führer."
(Translation: "Our glorious ideal is perishing, and with it everything that was beautiful, adorable, noble and good in my life. The world to come after the Fuhrer and National Socialism is not worth living in, and therefore I've also brought the children here [into the Bunker, to die]. They are too dear to continue in the coming era and a merciful God will understand, when I bring them salvation by my own hand. We've got but one goal left: devotion to the Fuhrer unto death.")
So you see, people dying for an idea(l) are hardly an argument for its veracity.
Besides, the borrowed elements were not plagiarized or deliberately added to deceive, but slowly made their way from one culture into the other, the way they usually do when people are in close contact with each other. None of the proto-Christians were the ones who borrowed them - that had happened centuries earlier, with Jewish folklore gradually adopting the image of an adversarial spritual entity.