In "Miraculous: Tales of Ladybug and Cat Noir," Hawk Moth (Gabriel Agreste) is driven by his desire to revive his wife, Emilie Agreste, who is in a comatose state. His actions are often motivated by his love for her and his wish to reunite their family. In the story, Hawk Moth seeks to obtain miraculous powers and use them to achieve his goals.
As for why he ascribed his soul to Adrien's mother, it's tied to his longing and desperation to bring her back. By attempting to harness miraculous powers and ultimately trying to fulfill his wishes, he believes that he can create a scenario where Adrien, his son, can see his mother again. This decision reflects his complicated character, as he balances between a loving father and a villain driven by obsession.
The intricacies of family dynamics and emotional connections play a significant role in the series, driving characters to make difficult choices in the name of love and loss.