I would be worried if you're intentionally giving pain to someone who does not ask for it. And you're crossing the boundary of consent.
A sadist uses pain for their pleasure & certain tantric energies. Just as a masochist uses receiving pain for pleasure too.
But a sadist also practices great control & self awareness. Never getting lost in the energy that would cause suffering.
Do you wanto cause actual suffering to a person (mental, physical, sexual etc) or do you enjoy the pleasure of giving pain.

This is a tricky area that must be studied & handled properly or you will really hurt your partner....which I would think would be a masochistic & there needs to be a great deal of communication between the two people.
Research & read. Answer your basic questions of why, how etc before you start to worry.
A sadist is not a bad person!