Why is it thought that the majority of natural selection is stabilizing selection?

The defendant, angered because a rival gang member had twice beaten him up after school, obtained a heavy lead pipe and waited in a deserted alleyway which he knew the rival took as a route home every day after school. when his enemy came walking down the alley, the defendant leapt out behind him and smashed the pipe into the victim's head, knocking him to the ground. the defendant then rolled the victim over and pounded his face with 15 to 20 heavy blows with the lead pipe, killing him. the jurisdiction defines first degree murder as murder committed with premeditation and deliberation. all other murders are defined as second degree murders. why will the defendant be convicted of first degree murder (as opposed to second degree murder)?