On Wednesday, August 7, 2013 2:50:02 PM UTC+1, mark wrote:
snip

Easier way is to take an empty syringe with a needle and inject air into a
suitable vein. Death by embolism, nothing to be detected and a tiny puncture
wound unlikely to be noticed. Only theory at the moment.

That's certainly been done in fiction - something by Dorothy L.Sayers?
I'm sure it's happened accidentally as well.