Why did Cobb perform Inception on Mal? ... In order to get Mal to kill herself and return to the real world, Cobb performed Inception on her, planting the idea that the world wasn't real in her mind. This worked, they killed themselves and escaped Limbo.

Why did Cobb and Mal go into limbo?

Cobb explains to Ariadne that he and Mal, his wife, ended up in their world-building limbo because they were experimenting with multi-dreams and Cobb pushed them too deep. He says they grew “old” together and eventually committed suicide on the train tracks to go back to reality.

Why is Mal evil in Inception?

She was there to witness Cobb's elevator of memories; Cobb revealed Mal's suicide to her; and she was there when Cobb finally had his realization that he had to let Mal go. Mal represented the evil in Cobb. She was a danger to everyone else that joined a shared dream world with him. He couldn't control her.