Who is responsible for Chillingworth's evil transformation? Why?
He is. He married Hester knowing that there was little "hope" for the marriage. He was an older scholar, and very lonely man, who craved human companionship. She was poor and looking for someone who might offer her other possibilities in life; she admits that she never felt anything for him. Chillingworth was able to overlook her obvious intentions and his needs and move into a relationship that had no future.
There are still lots of questions. Was Hester really sure that he had died? Did she enter a relationship with Dimmesdale based on this, or did she have a one time affair that ruined her life? Why didn't she hook up with Chillingworth when he came back into her life (other than the fact that if she had there wouldn't be a story :)). Did Chillingworth set out to ravenge the wrong done to him or to Hester?
At several points Chillingworth could have changed his future ... he could have refused to indulge his own needs and not marry Hester, or he could have forgiven her for her "indisgression" when he found her when he finally arrived in the New World.
He didn't make either of these decisions, and thus carried within himself the seeds of his own evil transformation.