An interesting character, my only concern is that the only reason presented for his apparent unstoppability is his intelligence. I think he needs a few more clubs in the bag to be able to kill that many people with impunity. Did he get mystical training ala The Shadow? Sell his soul? Find a mystical book of encantations? :) Go to Comment
Don't agree it is too fast for a poison - some real ones are ridiculously deadly.
It could also be non-magic if one believes in spontanous combustion being a real thing and this being a substance which triggers such an event. Exploding is a bit much, though.
It is, however,a bit too easy to create for the level of power it has. Whipping it up under duress seems unlikely as well.
Also, those with low BMI might not be killed :)
It does need a little more fat to it though, Moonhunters requests would help flesh it out.