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Fatuous One said:
Actually, sort of on that subject, I once read a doujinshi that had Shiki seeing the death lines for the entire universe... and I don't really think you understand that Shiki really cares for his friends and family. Granted, he'd try to avoid killing it if it involved other people dying, but if it was his sister or someone else (or many)... well... Not to mention if he went Nanaya.
Of course, I'm guessing that it would be the Akiha path in that case...
See, that's the problem with getting Shiki and the Beast Rune involved. The True Runes are literally alive, so if Shiki messed with a TR's death line, he'd effectively kill the rune itself.
Problem with that is, the 27 True Runes are the foundation of all creation. If even one went missing or was destroyed... well, let's just say you don't want it to happen.
I am aware that Shiki deeply cares for his friends and family. On the other hand, I don't see him running the concrete risk of committing universal genocide for the sake of a single person, no matter how much he cares for her. Killing a True Rune might well obliterate everyone, including Akiha, and even if reality remained in one piece, the energy discharge from the True Rune's annihilation would reduce an area the size of Australia to nothing more than a lifeless, blasted crater where nothing will ever grow again.
That's how powerful True Runes are. Hell, the True Fire Rune is one of the least powerful, and it reduced to ash three armies at once, scattered across an area the size of the Kanto. And that was only a tenth of its power at best.
High level mages with NORMAL runes can alter the landscape to the point of reducing entire mountains to dust and make it rain for weeks afterwards. And True Runes are the basic source from which normal ones draw power. That should give you an idea of why I'd bet Shiki would rather choose to not interfere, even if Akiha risks her life. The consequences of destroying a True Rune would be just too dreadful. They aren't worth a single life's salvation, plain and simple.