I wonder if there's a correlation between Crimson's inability to die and Ragna's inability to be killed. These are two separate things, as you can technically kill Crimson, he just doesn't die. Step 1. Kill him. Step 2. He dies. But Crimson doesn't ever get to step 2. Meanwhile, Ragna never crosses Step 1.
Wonder if the cost of Crimson's immortality meant essentially stealing someone's fated death.