Firstly we need to understand what is the meaning of sadist?
A sadist is someone who enjoys inflicting pain on others. Sadists like seeing other people hurt. A sadist is the opposite of a masochist, who enjoys being in pain. A sadist is all about hurting others.
Allah created world, Hell and Heaven and made laws to be followed accordingly. If a person do bad deeds will be punished for his deeds and if a person who does good deeds will be in heaven.
Eg: If in a class teacher takes exam; some children will pass and some will fail. So if a child fail ; will you blame teacher and call them sadist??
if a Judge gives death punishment to a rapist or murderer, will you call the Judge a Sadist? No, as Judge is giving punishment for their bad deeds and crime..
Therefore if some people go to hell for their bad deeds should not deprive people who do good deeds from going to heaven.
Allah is not sadist but Ar-Rahman and Ar-Rahim; Most Gracious and Most Merciful. HE forgive sins whenever a person ask for forgiveness and repent.
If Allah was sadist he wouldn’t have created Heaven for people who do good deeds and wouldn’t forgive sins of people and show his mercy.
In a hadith (sayings of Prophet Muhamamd) it is stated:
Allah’s mercy overpower his wrath (anger)