He only does it when he's alone.
Gamzee would never let on to a part of him having regrets about it.
He killed two trolls. Two trolls he called friends.
But - it was okay. That's what trolls do, right? It's in their blood. They're supposed to kill. They're meant to.
The rough nails, the sharp teeth, even the creatures they are raised by. They are all contributing factors, all things that make it easier for them to kill. All things that prove they were biologically made for it.
Part of him lit up, swelled with pride when he thought of what he did. The other part made his chest twist and ache and oh gog not again not again - he wasn't alone and he couldn't do this now.
Calm down, man, get your chill on. There's nothing you can do.
Another painful twist in his chest. Another swell of pride.
Ignore it.
Trolls really were vicious creatures. It was only so long before they would go mad anyway, before someone would snap, before they realized they aren't going to survive on this planet with every one of them within such a close proximity. They aren't going to survive eachother.
All they could really do is try, because after all, it's only natural.
It still didn't help that they would all look at him strangely when he began to shake and the indigo tears would spill from his eyes, down his cheeks and onto his otherwise blank face.
There was so much regret and hurt where there wasn't pride.
Best friend wasn't good with feelings jams, - not with Gamzee, anyway, and he hated piles of things - but he'd still feel a reassuring palm on his back whenever he couldn't control his trembling, and he'd still feel a hand on his scalp, pulling his head to a familiar place between the other troll's neck and collarbone where Gamzee buried his face and his body would wrack with silent, agonized sobs.
He was only safe for so long, with his hands clutching Karkat's shirt and his hitching breath finally coming to a calm before he knew he had to let go.
He wasn't entirely sure if his best friend truly cared, or if he was just keeping him under control, anyway.