Stevens had been taken over by BOSS, and tried to stop the Third Doctor and UNIT from shutting down the refinery.
The Doctor used a Metebelis crystal to help Stevens regain control of his mind. Stevens then supposedly sacrificed himself by triggering an explosion which destroyed BOSS and the refinery. (TV: The Green Death)
However, Stevens was not killed but in fact survived the disaster. He sometime after went on to become Director-General of the BBC.