How can I be sure that I am not sadist?

Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

It is compulsory for a husband to live honorably and kindly with his wife. It means he has to satisfy her sexual desire to protect her from Haram and should avoid enjoying with her through Haram ways, such as having sex in her back or deriving sexual pleasure by causing much pain to her, etc.
If you want more details, you may ask us again.
Allah knows best.