That doesn’t make the house evil. They’re not the nicest but we do see examples of Slytherins who aren’t all that bad. No one in Potterverse is perfect. Slughorn is the one who rallied the troops and who came back to fight Voldemort. Harry would not have recognized the Slytherins who fought, he was pretty against making friends with them (a flaw on his part). Slytherins aren’t going to be knights in shining armor, they’re the ones who say “We’re outnumbered, I’m coming back for reinforcement” and who carefully plan and scheme and go for what they want. There’s something to be admired in that, too.
I love being from the Hogwarts House that has the fewest dark wizards. I think the reason is that Puffs have produced few dark wizards is because being a death eater is counter intuitive to what Hufflepuffs prize. They don’t want or even care that much about power or victory, they are loyal and the only ones whose opinion truly matter is their friends, and they value fair play. If the game is played fairly that is true victory for a Hufflepuff
Today is the 32nd anniversary of their deaths.