Slytherin ideals aren't about being mean, it's about being cunning and sly, which Slughorn was to a T with his slug club shenanigans.
All of these are bad examples, or rather counterexamples.
Hufflepuff just takes "the rest." The misfits and outliers, the mediocre and the moderate. These can be great wizards and witches, or they can be utter shit. That's Hufflepuff.
Lockhart was exceptionally smart and talented, but he was a spineless weasel and gloryhound. He used his skills as at obliviation to alter memories and make people love him. He was a leech and a criminal, and inept at other magical skills, but he made himself rich, famous, and universally beloved while avoiding any actual practice or hard work. He was probably a very gifted student who figured out how to get other people to do his work and make him look good. That's why he was ultimately inept.
Pettigrew was a follower. No doubt he did not fit the mould of the typical Gryffindor in that he fell to corruption and betrayal, and managed to hide as a rat for decades. But remember, the sorting hat takes your choice into consideration. He likely wanted to be a strong and brave leader. He wanted to be cool like his friends, and he was not. He was small and scared, and always reminded of it. After school, his friends weren't there to protect him and make him feel special. He wasn't one of the "in" crowd anymore. He was a ripe target for a cult that brought him in and made him feel powerful again.
Slughorn out here being "nice" to everyone who didn't realize he sounds like somebody's racist grandma from the 60's every time he talks to or about Hermione.
"Some of my best friends are muggle-born!"
The sorting hat takes into account the member's own preference, too. It was about to put Harry Potter in Slytherin until he literally begged it not to.
So maybe not to confuse every first-grader-wannabe they should have named it The Suggesting Hat.
Also Harry even tells this to his son.
Yes, it's possible for the sorting hat to make sense as a concept and if we debate enough we'll get to the bottom of it.
Wait how was that hiffle not loyal?
I think that might be Zacharias Smith? He was the annoying Hufflepuff who was super skeptical and critical at the DA meetings.
