How did the Mind Flayer kill Billy?

Billy was slaughtered just moments after redeeming himself by saving Eleven (Millie Bobby Brown) from certain death, following a string of murders under the influence of the Mind Flayer. He had carried out the beast's bidding in the early part of the series, providing a steady stream of human hosts to be ingested.

How did Billy get possessed by the Mind Flayer?

A year later, in 1985, Lucas helped his friends confirm whether Billy was possessed by the Mind Flayer by trapping him in a sauna. ... It is unknown how Lucas felt when Billy sacrificed himself to protect him, Eleven, Max, and the rest of his friends from the Mind Flayer.