There are very few times when it is actually a good idea to kill bees. If you do have to kill bees, using a pesticide-free method at least helps a bit. Killing bees with soapy water is an option if it is needed.
When we mix soap with water it reduces the surface tension of the water. Water has very high surface tension, and this means that if a bee gets wet, the water does not easily penetrate the spiracles and drown the bee.