Beware of Pompous People
No one would deny that the series' obvious villains--Voldemort, Draco Malfoy, and arguably Snape--are dangerous. But it's the conceited secondary characters--like Gilderoy Lockhart, Percy Weasley, and Cornelius Fudge--who often do just as much damage as the true bad guys. Their self-satisfied bumbling leaves Harry and his friends exposed to the basilisk, an impostor Mad-Eye Moody, and Voldemort himself.
In the sixth book, Harry was wise to steer clear of the status-seeking Professor Slughorn, who wants to draw Harry into his clique. In Harry's world as in real life, serious evildoers are always a threat, but stuck-up people wreak plenty of havoc.