A homeless man who robbed and murdered an elderly woman in her Queens home — and made a sandwich in her kitchen as she lay dying — was sentenced yesterday to 15 years to life in prison.

Robert Marshall, 31, a schizophrenic, pleaded guilty to murder in January in the killing of 88-year-old Antoinette Russo.

In August 2004, Marshall broke into Russo’s Elmhurst home, tied her up and placed a mattress on top of her. He took a shower, ate a cheese sandwich and stole $20 before fleeing.

“It was an accident,” he said yesterday. “I’m really very sorry.”