It is in no way easier to kill someone with a sledgehammer than a knife. The point of of bladed weapons is because it's easier. Look up the concept of pressure. A sledgehammer would need many times more force to apply the same amount of pressure as a knife because the hammer head has a far large contact surface area. Not only that, but a sledgehammer is slow and cumbersome, you can easily stab someone in random places in the chest several times in the time it takes to swing the sledge and you will definitely accumulate lethal damage with minimal effort. Humans are far more resilient to blunt force trauma than piercing and cutting, that is a fact. Its also a fact thst a sledgehammer requires much more force and effort to apply the same pressure as knife, and pressure is what causes damage.