It's been a while since I've actually read the bits of the Bible about the archangels, but I don't think Gabriel is a tool (i.e. a jerk) so much as he's a tool in the sense of "the sword of God". He's a soldier following orders, and he doesn't even consider questioning them because they come from God, so it must be the right thing to do. If God says, "Take out Michael and the unborn child he's trying to protect," Gabriel will do that. It's a tricky mindset to get a handle on, particularly when the guy playing it in this instance has portrayed a lot of truly evil villains in his career.