Lord Voldemort showed me how wrong I was. There is no good and evil, there is only power, and those too weak to seek it… (17.31)
Spoken, of course, by someone on the "evil" side! If you've ever wondered how bad guys justify being bad, this statement by Quirrell is a fine starting point. To keep himself from doubting the "goodness" of his actions, Quirrell thinks only in terms of "power" and its absence. Of course, the absence of that power is defined by "weak[ness]"; having and manipulating power has positive connotations, while not exercising it – for whatever reason – has negative connotations.