L's eyes are downcast; he knew that there were two, but he feels strangely ignorant for only learning this now. They all have the responsibility to help each other fill in the blanks, of course, but if they were successors, doesn't it stand to reason that Lazarus should have been the first to know?

Not necessarily. What about this entire experience follows any semblance of reason?

"So... when you met Tony, he reminded you of this other boy," he says, trying to follow, already piecing together those sad and unpleasant implications. Then Nathan confirms it by describing Mello's temperament and complex, and he feels another pang of guilt on behalf of his Other. In addition to everything, he pitted children against each other in a competition to be adequate?

"I think that's a fair assessment," he says quietly. "I wonder if Near would have been callous if L was not. Just as you and I are similar, Near and L were, too. To an unsettling extent, even. Someone with the same goal would be driven mad by it if they wanted it enough."

In any cult, being close to the Leader is the be all, end all. Not achieving it is failure, exclusion, death. Mello burned.