Upgrading a Kicking to a Murder

[MetalMilitia]

Basically I know this guy and he a complete shit with absolutely no redeeming qualities. To top it off he insists on upsetting my girlfriend all the time by shouting abuse at her. There is a whole long back story behind it all but I really can't be arsed going through it. Suffice to say he deserves everything he has coming to him.

Anyway here is the problem, it's at the stage now that I'm about ready to kick the shit out of him the next time I see him, only thing is he's like 5' tall and about 10 stone. He's like a cross between a stick insect and a child. I on the other hand am 6'2" and 16 stone.

If I do engage him in a fierce melee it's going to be like taking candy from a baby... then scissor kicking the baby in the kidneys. In fact I'm actually worried that a single punch could seriously injure him just because he's so frail and wimpy. I mean I'm all up for giving him a good beating but I don't want to hospitalise him.
I would rather not end up in prison over this whole thing so what should I do? How easy is it to accidentally murder someone?