Q.1) We know that God has power over all things including satan. Then why does He not destroy satan? Wouldnt our lives be more happy if there was no satan in the first place?Q.2) In Karbala where Imam Hussain was oppressed, he was tortured badly. But why he didnt use his powers and kept calm as compared to other people?thanks

Walaikumussalam
1) when He asked for sajda he has not used His power as you know it comes in Quran la ikraha Fiddeen
There is no compulsion in Islam
He has given us free will and differentiated good and bad.
He never used power either against namrood and firaun and others .
This is our test and test of all human being to choose good with our own will.
2-
Not only Imam Husain as but in past other prophet has not used their powers but submitted themselves and completed their test successfully
And We will most certainly try you with somewhat of fear and hunger and loss of property and lives and fruits; and give good news to the patient,
Sura baqarah ayah 155
And when his Lord tried Ibrahim with certain words, he fulfilled them. He said: Surely I will make you an Imam of men. Ibrahim said: And of my offspring? My covenant does not include the unjust, said He.
Baqarah ayah 124
Wassalam