You have a pillow.
You have motive and opportunity.
Once you have a method for disposing of the body, everything should be straightforward.
Back when I was in the Navy, one of my petty officers talked about how, on her wedding day, her mother gave her a sewing kit and a baseball bat. The sewing kit, she was told, was a symbol of the effort it takes to sew a marriage together. Each stitch by itself was not much, but collectively they made a strong bond. “Okay, mom, whatever… and the baseball bat?” “If he ever treats you wrong, you sew him into his sheets while he’s asleep and work him over with the baseball bat!” Probably advice to apply to any relationship.