Old Friends Are Hard to Kill — Gary Towner

When an American CIA agent is ordered to terminate a fellow agent who has gone rouge, he is conflicted to learn his mark is his best friend. He finally decides the edict is warranted and fires the fatal shot. But the event leaves him devastated and he resorts to drink to ease his pain. Then comes the improbable news that that maybe his old nemesis didn't die after all. There are reports that a rebel leader in Guatemala leaves a trail of bloody ruthlessness identical to that his old friend exhibited. When they finally reunite, it's a fight to the death.