When there are two rhievess left, the last thieves will be left with nothing. Let's illustrate by using numbers. The smaller the number, the lower the ranking:
Thief 1 and thief two is left.
So thief 2 will suggest that he get all the 100 gold. Whether or not thief 1 agrees makes no difference to the outcome since thief 2 will surely agree, making it 50% agree.
So thief 1 will never allow that to happen. In this case, any amount of gold will prompt thief 1 to agree to the plan.
Thief 3 will use this to his advantage. So... Continue from here...
Repost question just in case forgotten
EDIT: Murder is absolutely forbidden unless following the rules (the highest ranking thief can be killed IF and ONLY IF the plan he suggests has lesser than 50% agree.)