I'm playing the UC and I'm looking for a mod that lets negative interactions actually give decent hits to Sims' relationships; as it stands right now, I have to spam "argue" between two Sims, as it only drops the STR by 1 point each time. I use SimBlender to drop Sims' relationships to enemy, but it doesn't always stick too well and I'd much rather play the decay out more. I tried searching for one, but all I found was an amicable break-up mod (which I may download, but I'm not sure if it'll work with the UC since it hasn't been updated in a long time).

You could try...
MATY - enemiesaccumulate.zip - Enemy status does not disappear over time if you ignore the relationship. (This can probably be found in the directors cut)
Cyjon - Argue Social Fix - Fixes reversed social values so mean sims enjoy arguments more than nice sims. Arguing always provides Social boost for instigating sim. Updated so argue always lowers STR and LTR rather than sometimes raising it.
Cyjon - Fury Initialization Fix - Prevents "furious" from ultimately improving relationships between sims

Oh and "The Fight Club" over @ MATY. Makes fighting matter and they seem to stay angry forever.

Those are perfect! Thank you so much.

Hey, @Nukk. I know this has already been answered, but I thought you might want to see these too:
Break Up Anytime - it doesn't reduce the relationship hit or fury
Fragile BFF - removes BFF status when sims become enemies
Always Defend Self - always poke or shove back the sim that pokes or shoves them
Always Insult When Insulted - makes sims insult back when they get insulted instead of crying
Lecture Enabled - allows you to have your sims lecture other sims anytime
No Autonomous Apologize - to also help keep the relationships down
Fight Anyone Anytime - I haven't tried this one yet though. EDIT: Tested and works fine in my game; enables fight interactions between teens and adults

A bit of a warning - if you have grouchy sims or those with negative reputations, these mods can easily keep them constantly at war with everyone else. Nice sims who find themselves at the receiving end will fight back instead of letting the slight pass.

I'll check those out too; thank you!