Maybe the first thief will suggest that he will kill 5 thieves then the 4 remaining that will not be killed will agree to the plan.After killing 5 thieves,the first thief was not contented he suggest again to kill 2 thieves then 2 thieves will agree again to the plan.After killing 2 thieves,there are only 3 thieves now but the first thief suggests again that he will kill 1 thief so that the two of them will have the 50% of the gold.After killing 1 thief the first thief was not contented getting only 50% of the gold then he came up an idea to kill the remaining thief so that he can get as much gold he will get.