I think Seamus will respawn, most shopkeepers do, he definitely will if you haven't yet completed "The Spines of America"

As for William I don't know, I didn't even know it was possible to kill him

Not bad because they respawn

Tf bro lol

I once drew on William by mistake. He instantly became hostile. I ran away and when I next encountered him he said something like

“have you calmed down now?” I think if you kill him he doesn’t respawn but don’t quote me on it.

I shot the gunsmith in the head in Rhodes when I robbed his basement (first time, by mistake, purposely anytime I’ve done it after) He just respawns after.