I think some of it made sense, though the way you phrase it suggests that B had no personality to begin with, whereas I'd argue that he did, but after all that time chasing after L, it was warped beyond any semblance of normalcy by his increasingly accurate attempts at near-continuous impersonation.

It's not so much that he's fooled himself, as his mental growth has worked out in such a way that he doesn't need to, because "psuedo-L" is the only personality he's ever managed to significantly develop. He knows who is, and changing what that is goes far beyond mere obsession.

And that's why you get an odd reaction trying to read him, because you'd think there must be something underneath that's him and not L, only he's been at it for long enough that nothing else has ever had any real chance to exist there.

Unfortunately it makes writing about him difficult, as artifice and lies are all you have to go on. You can't write about much else, because... well, what else is there?

Whatever real person was there went long ago, eliminated so he could be L, and we've not been told enough of his past to create an accurate portrait of that person.

As to the other aspect, I think your "Angel of Death" assessment is probably fairly accurate, (lol inexplicable death god eyes). If he ever did get a note he might find himself unable to change a person's death date altogether, since that would remove the justification in his mind.

And for the above reasons, I think if he ever had killed L he would either find himself incapable of functioning as a person any longer, or he would try to slip into the gap left behind, to make the impersonation as close to perfection as it could ever conceivably get.

...Reading all that back, it occurs to me that, if nothing else, you're probably not the least sensical writer on this page any more...