First, it is not certain that Zahard saw the fate of the 10 FHs. They are rulers protected by administrators. Even Gustang, who wanted to see Traumerei’s past, was forced to sacrifice himself.
Second, Zahard's order did not explicitly state that Gustang was to be killed. How to do it is up to Traumerei to decide. In addition, it was clear that with Traumerei’s strength he had a perfect chance to defeat Gustang. Gustang could hardly resist 3D Disconnection without using Blossom’s flame.
Third, the game is set by a third party (Red Dump), each person in the game has a different influence. Traumerei's loss comes from many factors: the branch heads betray, Enkidu, Urek and V appear. And Traumerei was the one who chose to commit suicide.
Fourth, Adori was unable to kill Gustang due to the immortality contract.
Finally, Traumerei was the most loyal to Zahard. So he has no advantage if his strongest ally dies.