In Episode 9, Ariel, Mal and Zoe steal the medicine and Jayne guards Simon and River as they sneak into the hospital. Previously Jayne had expressed some disgust at River staying on Serenity, since River slashed his chest with a large knife. What ends up happening is predictable--Jayne betrays the Tams, therefore betraying Mal--which he knew would happen someday.
But Mal could easily have taken Jayne with him and left Zoe with Simon and River. Why didn't he do that? Did he intentionally do this because he knew he would be able to get Jayne and the Tams back safely this time, or to ensure Jayne's loyalty?