* Rwanda's ex-spy chief latest victim in trail of killings
* U.S. says "troubled" by murders of Rwandan exiles
* Rwanda's Kagame unapologetic over right of self-defence
By Helen Nyambura-Mwaura and Jenny Clover
JOHANNESBURG/KIGALI, Jan 17 (Reuters) - The United States says it is deeply worried about threats made by Rwandan President Paul Kagame against political opponents after one of ...