gendanken, how about HF for killing someone? As an acid, it is very weak and doesn't even chew away viciously at your skin. However, the "symptoms of skin exposure to dilute HF are not felt immediately, but exposure of less than 10% of the body to it can be fatal, even with immediate medical treatment" (Wikipedia). Basically, you'll feel a burn but later your bones will start to decompose, which I understand is VERY painful. Hydrofluoric acid can be circulated through your body too, so if you're lucky then you'll only have to have a few limbs amputated (depending on dosage).
Sooo... spray someone with some of this and see what CSI thinks about it... Course this is just for killing, not covering up a kill---just thought I'd add to the discussion. There are bacteria too which are just as effective... Something the KGB use(d) and you don't need more than a pin prick to deliver death. I think I read in a newspaper of a Russian politician feeling a slight sting of a needle (turned out to be the point on an umbrella) while getting off a bus, a large lump on his leg the next day, and a large tombstone on his head the next (may have been 2 or 3 days actually..). Anyone heard anything like this though?