The cowboy did have sympathy, as hard as it was to believe. After all, since three-quarters of his body was made out of metal, what stopped his brain from being made of metal too? It was common for people to ask that (that is, whenever they even got the courage to approach). He liked pretending those questions didn't make him upset.
It wasn't his fault that the IPC destroyed his planet. Or that he could "look threatening" to others.
He tried his best to be empathetic with others—to make his presence remembered well by whoever he tried to help. But it was hard to act kindly when nearly every day he was met with harsh glares from others for simply existing.
It was even harder to sympathize with others when no one ever sympathized with him. Never did he receive the treatment he constantly tried to give to others, which led to every act of sympathy feeling emptier and emptier, as he received nothing but indifference.
He knew he was an isolated case. It was not every day that the IPC destroyed a planet, after all.

That's why he felt revolted when he discovered yet another planet had been destroyed. But maybe he could save someone. Maybe he'd finally get that empathy he'd always chased. And on the off chance someone had survived, Boothill visited said planet.
The chances of someone even being alive were—really, none. Zero. And even if they were alive, it was likely they'd be all ripped apart, limbs sprawled around, bones broken, or crushed. Which is exactly the reason as to why he was confused when he found you. Alive.
"Oh fudge kid!—"
He held his hat as he ran towards you, grabbing you by the shoulders. You appeared to have some mild burns—probably from the explosion, but for the most part, you appeared to be just fine.
"Y- Yer... Ya're okay, yeah?... Just- Just... just a lil' burnt—"
Boothill asked as he brushed off some dirt from your face with his cold metal thumb.