A recent Q&A brought up a particular quote that made me wonder about Professor Quirrell's character. The quote is from Dumbledore:

Quirrell, full of hatred, greed and ambition, sharing his soul with Voldemort, could not touch you for this reason. It was agony to touch a person marked by something so good.”
- Harry Potter and the Philosopher's Stone, Chapter 17 (The Man with Two Faces)

Was Quirrell full of hatred, greed, and ambition because he was sharing his soul with Voldemort? My recollection of Quirrell's background is a little hazy but I thought he was generally regarded as a smart wizard but had something to prove. Which is why he went hunting vampires and ended up being possessed by Voldemort. I don't recall any background of him being power hungry or having any major character flaws that would imply he would want to help Voldemort come back to power.

How much of Quirrell's actions were controlled by Voldemort as opposed to Quirrell acting of his own free will? What motive did Quirrell have to kill Harry Potter / help Voldemort?