I was a candle

If devils can be killed, then it certainly isn’t easy! They’re tougher, stronger and faster than any Londoner, and probably more technologically advanced to boot. There’s already been one attempted invasion of Hell, so surely someone out there knows something about their mortality.

Let’s ask the Regretful Soldier!

“No! You weren’t there! You don’t know! They took us! They took our souls! I’m as brave as any man alive, and I’m not going back there. You can’t kill a devil. You can’t kill a devil. You can’t kill a devil.’”

oh

Maybe we should get a second opinion?

What does London’s public have to say?

The mortality of devils is uncertain. A few bravos have claimed to kill one, but half the time the devil in question turns up later. Some well-known devils have been missing for years. But it’s possible that they were recalled rather than buried. But there are stories. One story says that they can be drowned, but not burned. Another says that beheading will end them. A Gin-Ravaged Gonoph claims to have seen a devil’s corpse, but his story is a crazed rant about fire and bees.

Interesting, but inconclusive. To get anything comprehensive, we’re going to have to go straight to the source; let’s ask the devils themselves!

“Ah, Death. We holiday on the River, did I ever tell you that? If we cross, we rarely return, but we find the airs of that place restful. We have died, of course, all of us, at least once, or we wouldn’t be citizens of Hell. And so we have some experience with Death. We have our treaties and our accommodations.”

“We dislike Death. We find it troubles our timetables. Yet when one falls, another rises. The same one? Hell is unnumbered, my dear. And so who can say? Did I ever tell you my name? Why do you think that might be? If I said to you that Death visits us as the bee visits the flower, what would that mean to you?" 

"There was a time when Death feared fire. And then we did not fear him. That time is long past… but it is very difficult to burn us. Let me stoke the fire. No more of Death. If I die, will another rise in my place? Or will it be me, the next instar? Perhaps you would grieve. Perhaps. You should leave flowers on my memorial stone, then. Red roses.

Bees? Flowers? Fire? Devils can talk a lot and say very little. There’s only one way to get a conclusive answer here. Fetch a knife!

"It’s a hard fight. Terribly hard. At last he falls, and the light goes out of his eyes. You stand over him, panting… and his flesh begins to crumple, like paper. 
It is paper. The devil is paper! He crumbles and shrivels. Something crawls from his mouth and takes wing: a bee? A lamplighter bee? 
Applause. All around you on the roof-tops, devils rise to their feet. Their eyes glow in the dark. Some of them throw flowers. What’s happening here? Is this devil dead, or was it some trick? You turn and run as the first devil drops into the street before you…”

well

Conclusions? It probably is possible to kill a devil. You’ll just need to bring a butterfly net.