Just look at the way Draco is treated by his own father. The scene in knocturn alley (Chamber of Secrets) when Draco accompanied his father to dispose of dark items. Malfoy berated his son for making lower marks than Hermione whom he called a mud blood. His comments were not encouraging or loving. We need to look no farther than the Father to explain the actions of the son. He treats others the way he sees his father treat others -- the way his father treats him -- the way Voldomort treats his followers.