With great power comes great responsibility; what are some secret powers Superman could have that are so dark that he’s too moral to EVER use?
My entry: super-seduction. Superman could turn any woman he chose into his drooling love slave if he wanted.
Relatedly: using his X-ray vision to look through the clothing of attractive women (or men, for that matter). Note that this was teased at in the first Christopher Reeve Superman movie, when Lois asked him if he could tell what color underwear she had on.
Hey, that may already be a power he possesses. Ask Lois. 
He can just puff a little air (subtly in other words) and blow all of the clothes off of someone.
How about necromancy?
Supes did use the speed-grope on Lois in Superman 1978.
He could use his superspeed to mess up every outdoor chess game in the world.
He could cause makeup become crumbly and also cause extremely bad breath. You know, super Kal-El fragile lipstick extra halitosis.
Go to your room!
Superman can fry you with his heat vision. For a new, evil superpower give him microwave vision, so he can cook you from the inside out!
But that could be used for good too. Like if you went on a camping trip with him, you wouldn’t have to carry a camp stove.
It’s hard to think of powers that can ONLY be used for evil purposes. Even super-seduction might be used appropriately to defeat a female supervillian. How about super-Holocaust, that causes the immediate death of all members of a selected ethnicity?
Why should powers of seduction only work on women? Presumably some number of them are lesbians or asexuals. Men wouldn’t be immune either.
Any sort of mind reading or mental manipulation is pretty morally dodgy. In Superman II, he causes Lois to forget having discovered his secret identity. It’s presented as a kindness to remove unpleasant memories, but he makes the decision to mess with her mind for his convenience.
Wonder Woman’s lasso of truth kinda strikes me the same way. If I want to keep something secret, that should be my decision to make.
No, that would still be evil.
Why would Superman need more or different superpowers to be evil? He’s already almost completely invulnerable to harm, super strong, has super speed, and has x-ray and heat vision. He’s the most overpowered superhero there’s ever been. If he wanted to, he could rule the world as an cruel dictator and do anything his evil heart desired, with the powers he already has.
Does the lasso compel someone to tell the truth or does it compel them to answer against their will?
The way I understand it is if you are asked ‘where did you plant the bomb?’, you can’t say the park if you put in the train station. That is a lie. However, ‘I don’t want to tell you’ might be a truthful answer…
AIUI, though this has undoubtedly varied over the years (as has most questions about superheroes’ powers and gear), it not only compels the person to speak truthfully, but also compels them to answer questions and not attempt to be evasive.
Give him the power to cause sickness or dementia. That’s evil.
Very limited teleportation of small objects. Could summon lego under your bare foot or a coin into an artery.