Well to get more in depth, it's less about the martyrdom per se, and more about the spiritual martyrdom with progressing past the "harsh god" of the old testament

The old testament god was infamously brutal, and mass killings/sacrifices of potentially innocent people were condoned, like the first born Egyptian sons dying for the crimes of their leadership

Let's remember that at the time it was fully expected for followers of god to engage in regular sacrifices of animals as offerings as well, with some more fanatical sects of levantine faiths engaging in human sacrifices, alongside other very strict practices more focused on ritual than about the well being of the followers

The new god of the new testament reformed the harsh world, and offered up himself in a flesh and blood form, through his own son Jesus Christ, as a sacrifice to show his love for humanity and change the world

Hence "Jesus is the lamb of god", lambs being the animal traditionally sacrificed for rituals by human beings