What is the real purpose of Fan Zeng's last hint to Xiang Yu at the Hongmen Banquet?

Using double agents to kill Han Xin, Sean and Xiao He. Because he knew Xiang Yu wouldn't open the kit, Liu Bang finally opened the kit. As long as Liu Bang does this trick, he will kill all three people. Liu Bangming knew that this was a simple double-faced spy, and this move just gave Liu Bang a reason to kill these three people. Killing heroes for no reason requires a good reason.