It's a darn sight lower than leaving him unguarded in the garden.
Same as killing the Joker. It might well not stop him permenatly, but I think even Mr J would find it ever so slightly harder to escape being, y'know, dead, than Arkham Ayslum's revolving door system...
And if narrative casualities always says "screw you" then nothing you ever do ever matters in the first place, so why bother?
But I, personally, don't believe that, anymore than I believe that all the preportedly unkillable concept personifications (e.g. Unicron, Galactus, most demon lords blahblahblah...) that continuously spout off about how they can't be defeated are really unkillable.