Shredder seems a bit TOO obsessed with destroying Splinter.

[Atren]

Originally Posted by The Big Bad

Why the heck not? Isn't this the guy who's constantly threatening to kill his underlings? As far as the Shredder knew, the mutation made Xever dead weight--he couldn't fight, and wasn't in any position to be able to remain informed about the streets, which was his selling point in the first place; all he was good for was dissection and help in figuring out how he got mutated, except that as we've seen, the Shredder couldn't care less about that. And yet, the Shredder kept him in a fish tank, as if he knew that if he waited long enough, the writers would eventually drop into his lap a way to put him back in play.

One more detail that might be overlooked by some is if you kill who replaces then? Until Shredder finds competent replacement he is relatively safe..until replacement found. That threat is what should make Xever tremble and try harder in future -- as he might have until then to show his worth.