Mine are usually very nice and upright citizens, too, so having a Sim like that is great fun! She's not evil per se, just very nosy, and since she's in PR, she figures having leverage over others can't hurt. And the secrets are soooo much fun. I actually use one to explain away something that has been annoying me for at least two years at this point: I have this one Sim couple, happily married, both have the loyal trait, their romance bar is as pink as it gets, they treat each other lovingly... but from the very start, they were anxious about having betrayed the other. The option "confess betrayal" pops up constantly; one of them is always out and about stargazing with others when I'm not playing their household. At one point, she even went off to stargaze while I was playing their household, leaving her wife with their fitness club while she met up with some random NPC to do some stargazing.
Now, I still don't understand why this keeps happening. It doesn't make sense, personality-wise. But now my nosy Sim has discovered her secret: she wants to have lots of kids with at least five different partners! This is kinda perfect because it allows me to roleplay that annoying bug, or glitch, or whatever it is into "Of course she's flirty with everybody, she wants to do the 100 Baby Challenge!" I think at one point, I'll just have both of them confess that alleged betrayal, then send them to marriage counseling.