Who Poisoned Khal Drogo?

A witch, Mirri Maz Durr, who pretends to be a healer, tells Dany that she could treat the wound and make Drogo well again. However, she practises blood magic on him and the wound becomes fatal, enough to take Drogo's life.

Why did Daenerys Targaryen kills Khal Drogo?

After initially tending to Drogo's wound, Daenerys convinced her to use blood magic to save the man's life. Rather than helping him, Duur cursed Drogo so he would remain in a vegetative state while also preventing Daenerys from giving birth to a healthy infant son.

What happened to Khal Drogo in Game of Thrones?

Khal Drogo died at the end of Game of Thrones season 1, mercy-killed by his wife, Daenerys Targaryen.