Truly evil people were not the kind you could see through at a glance. When they revealed their true face, either they had already gone mad, or there was nowhere left for you to run.
Before Dumbledore renamed the Dark Arts class Defense Against the Dark Arts, Expecto Patronum fell within the realm of the Dark Arts. A happy memory could be a birthday party with friends, a trip with family, or even robbing Gringotts or blowing up the Ministry of Magic. What Patronus was summoned was directly connected to the wizard casting it.
Throwing a rabbit into a nest of snakes was called feeding them. The members of Harry Potter's "Dumbledore's Army" mostly summoned mammals or birds. If Slytherin got to choose, summoning a Dragon would naturally be the most impressive. A three-headed hellhound, a Nundu, or a snake would do as lesser alternatives. But when someone else summoned a Dragon and you summoned a rabbit...
Slytherin's Dark Arts class was different from those at other schools. Darkness was power, and calling them white wizards was an insult. Dumbledore excelled at fire magic. His pet was a Phoenix that could be reborn in flames, so there was no need to worry about burning it to death with fire magic. But while he understood the water-based magic of Expecto Patronum, his mastery of it might not have been particularly profound—at least not as good as Slytherin's old bat.
The place where Harry Potter saved Sirius Black was beside a pond in the Forbidden Forest. After using the Time-Turner to return to the past, the Patronus he summoned the second time was even stronger than the one he had cast the first time. By then, his physical strength and energy were already worse than before, but human potential was limitless. Squeeze