User blog comment:MilitaryBrat/Lord Voldemort vs Darth Vader/@comment-181289-20111217025951/@comment-4248317-20111217094451

@Swg66 As a Harry Potter fan, I can say that worldess magic is not limited to more simple tasks/spells; just about any spell can be performed nonverbally, depending on the skill of the wizard in question. Dumbeldore demonstrated this in book 6 when he created an enormous ring of fire without saying a word.

Also, according to the Harry Potter wiki, Bellatrix's spell she used to kill the fox was in fact nonverbal.

In regards to your question as to why they don't just use nothing but nonverbal spells, the wiki also states that most spells appear to be less powerful when not spoken aloud; I assume the fox was still killed because it was weaker than a human.