Why didn't Qui-Gon kill Obi-Wan?

In Phantom Menace, Qui-Gon finds the Chosen One (Anakin) who he believes he has a duty to train. The Jedi Council won't let him train Anakin partly because Qui-Gon already has an apprentice. so why didn't Qui-Gon kill Obi-Wan and free up that position for Anakin to take?