I think that this might be a good response:
"I feel sorry for you.
What an empty life you must live to have made you such a bitter, twisted malicious person.
What was the intention behind what you did (apart from the obvious one, of course)?
Did it give you a thrill when you put it in the letter box? 2 first class stamps as well, no expense spared, was there!
But what made you even think of doing such a thing to someone you have never met, and who has never (knowingly) done anything to you? Perhaps it's a hobby, if it is then I'm sure there are better ones you could find. Have you considered pulling the wings off flies?
Anyway, I must go now, there are nice people out there who I would much rather spend my time on.
Lots of love, (because you need it)
Phoenix