In graffiti terms, to hack is to take either a pen, pencil, or marker and draw a clear line through another's hard work. Why one might choose to hack another's art is usually justified because the artwork was tagged by an opposing artist or personal rival. In some cases, the art just might not appeal to the "hacker" for numerous reasons.
"No reason. It's just that Paul made it; you know how he pisses me off."
*A term referring to a cheater in a game of CS.
"Isn't it protected by a password and firewall?"
"Yes but...there are ways. ^_~ "
-Otacon & Snake conversing about trying to find out about the PAL system
"That guy's too good. He must be a hacker. Damn him!"
-me, getting my ass kicked by someone who is just too damn good at CS