How do you make orcs truly evil, in your campaign, so that it punches your players in the gut?
You don't give the orcs pseudo-Cockney or -hillbilly accents. None of them crack recognizable jokes, and if any do "joke," the other orcs laugh in the sort of laughter you'd use to scare the hell out of a ten year old ... no intonation we'd associate with mirth. They don't speak broken English, they don't grunt or scratch themselves when they speak, they don't refer to themselves in the third person, they don't dress in rags, they don't carry bizarre looking weapons, they don't look like Warhammer rejects, they don't have oft-repeated catchphrases or shout "WAAAAGH" in unison, and they don't wear spiky armor – in short, banish everything that gamers and filmmakers use to provide comic relief. At all times when they are in play, you compose your face to sober mien, and you keep all jocularity out of your voice.
Sanitize nothing. You don't gloss over the atrocities with a "They did everything possible to the village." So what? Yeah, that's what bog standard orcs always do, right? Kill, rape, pillage, burn, torture, cannibalize, we've heard it in half a hundred campaigns, yeah, they're evil, we get it.
No, you hit them in the faces with it, and you hit them hard. The party's walking down the pass towards the village, to see the raid in progress ... and the orcs in the party's LOS are visibly sodomizing a nine-year-old, whose thin screams they can hear on the wind. And in the few minutes it takes the party to run to close the distance, the orcs toss the limp body to one side – where you graphically describe a warg-type biting chunks off of it – and start to rape the next one. Even after the orcs are defeated (or run away), you present the group with more horribly wounded and dying people than the party has healing powers to fix, and they're not dying quietly ... screams of agony, horror, the whole nine yards. People on impaling stakes, still alive. Village elders with their eyes eaten out of their faces, scrabbling in the dirt with the bleeding stumps of their fingers.
And if any of the players wants to crack jokes or make light of it? Treat it as if it were done IC, whether or no ... and that's a dandy opportunity for the trusted NPC/henchman to go chalk white, before spitting in the face of the offender and hissing, "You sick bastard," then hoisting his pack and marching off back the way the group came. Nothing will persuade the NPC to change his mind, short of the immediate execution of the joker as being just as depraved as the orcs are.
Me, I'd never want to take a campaign in that direction, nor have the stomach for it if I did. But if you do, that's how you do it.
Additional Ideas (1)
I took a step back and rewrote them entirely. There is still emphasis on destruction, and some on chaos and evil but now there is more texture.