Why did Henry VIII chop peoples heads off?

Henry VIII did not, personally, chop anyone's head off.

Two of his six wives were tried, convicted and executed for treason and other crimes.

See the related question link below. Many other people were also executed in Henry's time - it was quite a normal way of getting rid of people whose faces did not fit. One estimate has it that as many as 70,000 people were executed during his reign, mostly by hanging. The more "civilised" beheading was reserved for the aristocracy.