Everything I can think of in order of what's immediately possible without forgoing certain bits. The bolded are stuff you need to do before anything else.
Break up with Bess/Ben. The most monstrous thing you could do.
Sell Hand So after talking the AI down. Or destroy the AI. More classy to not leave loose ends.
Let Tarkus get blown up.
Don't promise Embry that you will avoid telling anybody.
Kill Taivara.
Do the Bothrioc quest, then go to Uveto to get the Fake Counteragent from Lessau. Attend the bothrioc meeting, and spray the fake counteragent. You knowingly prevent a species, however dangerous they may be, from being put up with the likes of the Dzaan, and basically steal a legendary artifact from them that their ambassador gives to you willingly, despite what you did.
Let the fungi queen burn.
Let Fazian be taken by the ants in exchange for the bribe.
Bag the queen instead of letting her run to Mhen'ga or a Kui Tan world. Arguably the worst outcome since you would be dooming both reds and golds to die on Myrellion, with nothing stating that either would be safe.
Fuck the Water Queen with penis.
Hide your identity from Shade, then do the Kara Quest mission and be a complete dick to Kara. After which, Kara will kill Shade, meaning you get your half-sister killed by proxy of your actions.
Impregnate Briha, the red ant deserter. Then blow up Myrellion. You killed your lover and your unborn child in one fell swoop.
Steal Roz's core. Arguably worse since she would have a harder time resisting being a bimbo, meaning somebody stealing her armor becomes far more likely, and it's all downhill from there.
End combat without fucking the rat raiders(this decreases their friendship with you, but let's face it, you wouldn't want to be friends with a Robin Hood group if you were a true jackass).
Complete Syri quest, but let Valden die as to make the trip pointless and Syri extremely upset.
Kill the Frostwyrm on Uveto, helping in dooming the race to extinction. (You should then go to Korgii Hold to make armor from the scales)
Let Korgii Hold be destroyed by the Milodans when Ula asks you for help(and impregnate her for the funny as well). Or, if you want to exploit Kiona(you can basically buy savicite from her for less than base price by selling her own piercings to her in exchange for savicite and not credits), instead sentence Eitan and his tribe to permanent mining life.
Kill Zaalt.
Burn the plantation and join the bees during the Plantation Quest. Then win the Pollen Festival and impregnate Quinn. Tell her that you don't approve of her using herbs and then proceed to bail on her for the entire pregnancy.
Feminize Del.
Refuse to be a father or provide money for Zheniya, the call girl on Tavros.
Refuse to take the kids from the zil girl or raskvel girl on Mhen'ga and Tarkus respectively.
Prioritize the probe instead of your cousin... pretty much the only thing I could think of on Dhaal. Besides the party that is. The options that are worst for the planet should be self-evident.
Have Tittyblossom Bizzy meet her mother? I haven't tried it myself as I don't like being a dick, but I think that meeting would not go well.
Lay parasitic tail spawn wherever you can, be it cunt tails or cockvines. Cockvines especially if you want to be a public menace; if those are spread around, I'm pretty sure the idle obscenity is going to cause at least a few minors to be mentally damaged or ruined. Unless the cockvine has some sort of instinct to not do stuff at certain times and places, as well as people. Point is: Cockvines aren't painful like Cunt Tails, so there's the chance that they can attach themselves to someone in their sleep.
Make Sera a sub and encourage her to be a tart. Make sure to do every option that ruins her, remove cock, regen hymen, change to pink... you may not be able to impregnate her, but you crush her dreams entirely so yeah. Or just never train her to be a sub, and just ream her ass since she'd never grow to like it if you keep fucking her without her arousal being high.
Be a jackass to the Love Starz after they split for a bit.
Be a jackass in Tessa's invite to the wedding.
Hard reject Chaurmine.
Make Vark subby.
Fuck Beatrice.
Reject Fisianna entirely(you monster).
Don't wait for Paige when making her eyes come back. Or just don't fund her eye surgery. She'll have years, after all.
Turn Penny into a Bimbo without consent.
Turn the goblin pirate into the authorities. Along with every other one.
