People change.
Constant experience, constant accommodation, constant reflection and introspection enable continuous growth and change. Sometimes progress is made in a positive direction, and sometimes in the opposite direction. On the whole, basically everyone changes; it's only a matter of to what extent and in which direction.
Some people do indeed undergo drastic personality changes after a certain stimulus, as if they became another person overnight. The gentle become devils who kill without batting an eye, and the most unforgivable villains reform themselves completely. Hence the saying, "Great men can change dramatically, lowly men remake themselves, gentlemen can transform like leopards," but no matter how intense the change is, humans cannot become "something else."
