"Maitreya naturally didn't want Cundi to die, so He hadn't harbored much malice towards the Three-Burial Mage.
He merely wanted to make progress like Tathagata had.
Moreover, His reason was more than sufficient:
"I had always assumed that at the Supreme Unity stage each step was arduous, until the Tathagata refined Master's Broken Palm, then I realized there was such a shortcut. So it seems, as long as the Saint is willing to sacrifice just a little bit, it could benefit countless disciples. The Saint--is truly too selfish, especially Heavenly King Yuan Shi."
He Himself, after all, had already been promoted to Supreme Unity.
Heavenly King Yuan Shi always complains that none of his twelve Direct Disciples can reach the Supreme Unity stage. Initially, He felt sympathy for him, but now it seems that Heavenly King Yuan Shi is just too stingy. Always making excuses, never willing to sacrifice Himself.