As always when discussing the usage of English words, it's pleasing to look at Webster's 1913 dictionary, which helpfully includes this definition of "tool":
> A person used as an instrument by another person; -- a word of reproach; as, men of intrigue have their tools, by whose agency they accomplish their purposes.
> A person used as an instrument by another person; -- a word of reproach; as, men of intrigue have their tools, by whose agency they accomplish their purposes.