A sadistic person is one who gets pleasure from the pain and misery of a human being.
It's actually quite strange. Speaking from experience, sadistic pleasure is the most euphoric pleasure you can get sexually. Honestly, there's nothing more satisfying, if you are in the right mood, than seeing the horrendous suffering and humiliation of a victim. It will make you feel like god. Like in control. It will make you taste the victim's blood, and it's sooooo sweet! After being accommodated with sadism, no typical porn can even satisfy you. Regular sex is sometimes just not strong enough. You'll want to tear the pretty girl apart!
WARNING: Sadism is an EXTREMELY dangerous practice to engage. If you don't have enough control over your mind, it can draw you to many horrible things such as crime and homicide. If you ever watch the biography of many serial killers, they all start out with a fetish in what they call "violent pornography." If you don't have enough mental power to keep sadism into perspective, it can pull you into awful illegal commitments, destroy your life, embarrass you, make you feel guilty, and make your mood generally unstable.
Sadistic individuals who can control their will are the most incredibly bright people. These are the very CEO's, the lawyers, the engineers, etc behind the scenes. They can be wonderful people. As long as sadism is well controlled, it can be a pleasurable experience, especially if the victim, the masochist, actually enjoys it.
Sadistic pleasure in context of kink is fun, even for a girl while being hurt. Just don't let it grow to be something illegal or awful.