Montague believed Mayeur was trying to kill him, so he killed Mayeur to prevent that. However, Mayeur was able to survive by transferring his mind into Brother Tomas. He used the Quoth to overlay his personality on other politicians to increase his influence. Others to whom he mentally migrated were Hubert Henri and Jules Balmarian.
After Montague discovered his body-swapping trick, he killed Mayeur's other potential hosts. Mayeur was finally forced to transfer into his last host, Pierre Duval, where he was incapacitated by Roz Forrester and the Seventh Doctor. (PROSE: The Death of Art)