10, no, only three companions have ever been killed. After not having hid on earth for long Turlough got bored, stole the Brigadier's car (the Brigadier was teaching at his school at the time) and crashed it, while having an out of body experience he makes a deal with the black guardian to kill the Doctor, don't ask me why
10 the Brigadier is a military person who was in charge of the Doctor when he was stranded on earth in his third incarnation
Turlough's first like four episodes have the subplot that Turlough has been hired by the Black Guardian (not entirely sure where he comes from) to kill the Doctor. But truthfully Turlough is a horrible assassin