@Zeonll I'd agree with Thelegendarytaeyamada. You have to take into account the weight of the sword, the pointy edge and its speed. The swiftness of the blow increases its force quadratically. Twice the speed means 4 times the force ! Thrice means 9 times!
Swey's sword seem heavy and pointy. He's a trained knight and applying some strength, a skull would be pierced relatively easily. You overestimate how sturdy a skull is. People falling backward can fracture their own skull under their own weight.
Beheading someone is somewhat harder, but with a sharp blade, strength, speed, weight and the right angle, it's doable in one move.
But it wouldn't be easy to do horizontally.