Once we can control fully developed adult brains, at the level you're suggesting for this thought experiment, that power will force us to reconsider criminality as a mental health issue — even if the personality disorders leading to criminality happen to be harder to fix in adults than boosting of IQ.

But note how I phrased that: Being able to rewrite the DNA of killers etc. to make them smarter, in the absence of influencing developed adult brains, only makes their descendants (in the strictly genetic sense of the word) smarter.

At some point in this century, and probably sooner rather than later, we're going to be able to cost-effectively write arbitrary human-length genomes. Simply printing a custom genome will likely happen well before it becomes possible to safely rewrite live adult genomes, which is itself a different task from understanding, controlling, or safely re-activating in adults, the developmental pathways that lead to healthy growth within a brain for things as vague as "lust", "empathy", or "intelligence".

But to your previous question, "At what point does it get silly?": at some point, we're all made of atoms, and if we had a level of control over matter as in fictional narratives like The Culture or Star Trek, then (modulo weight changes) all your atoms can be rearranged to turn you into a copy of me, or anyone else on the planet, or any other species including fully customised not-found-in-nature varieties.

I'm reminded of a cover of a Monty Python song:

> Oh, I'm a lycanthrope and I'm okay, I romp all night and I sleep all day.

- http://web.archive.org/web/20080509070613/http://www.swampfo...

Silly is fine :)