Tommy realized he definitely couldn't outdo her. Once the "Serpent's Offspring" ritual was over, his Path potential would be drained away.
Thus, after plotting, he completed a murder by studying the ritual. He cursed his sister with zoophilia, had her mate with a bull, and ensured she was accidentally discovered by others. She chose suicide afterward.
But in reality, she failed to kill herself—Tommy used another woman's corpse to disguise as her body. He raped and sacrificed "Rowetta" along with his two younger brothers to summon the Maze Demon.
His father didn't realize this was Tommy's doing, so he bestowed the Lohar name upon him. To prevent his father from realizing later, Tommy dragged him into the labyrinth to die of starvation.
The Supervisory Bureau suspected him at the time, but they couldn't find any evidence whatsoever.