Killing her is no big deal... Having sex with the corpse is wrong, though.
Since this is Britain, I predict he will be out in 4 years. That ought to teach him how wrong it was.
Oh, well; let's just issue him an antisocial behavior order, since he already admits he knows it was wrong.
That, plus some public service time, and a good stiff finger-shaking at him ought to do the trick, by Jove!