When I looked it up online, I saw that someone on Yahoo! Answers said, "A 'tool' is someone who does not make decisions or statements for themselves. Rather, they parrot a party line and act more on what others may think of them than on what is right." Hmm. What do you think, or what would be your definition of someone who is a tool?
I feel like "you're a tool" should be the complete opposite of an insult. I think we can be tools for good, for our Heavenly Father. It looks like in all of the verses in the Bible and other scriptures the word "tool" simply means a tool that you make something with. But I love the writing that uses the word "instrument".
Lord, make me an instrument of thy peace. [prayer of Saint Francis]
we have been made instruments in the hands of God to bring about this great work [source]