Question: are there any deadly Pokemon attacks?

Earthquake or magnitude could do some serious damage to anything

 

If we're talking realistically, there are tons that could kill a mon. In the context of the official series? Nope. Death through moves is not a part of it.

 

Death through moves don't seem to be part of the anime or games...however, the early Pokémon Adventures manga...yeah, maybe you should check that out.

 

There's been a few times in the anime where it seems implied that a Pokémon could die if it was mauled by wild Pokémon, and its trainer didn't get it medical attention in time (I think that was part of the plot of the "Squirtle Squad" episode, but I'd have to check...it's been a while since I've seen that episode). Then again, the anime seems to flip-flop on whether a Pokémon knocked out in battle needs to be rushed to a Pokémon Centre, or just needs to be returned to its ball with a quick "You did well, now take a rest.", depending on what's more convenient/dramatic.

The anime also implies that Pokémon could die, or at least become seriously ill from untreated poisoning - although given that its a series aimed towards children, they never outright say Pokémon are in danger of death, just that they're "in serious trouble" or something like that. There's an episode where Brock is forced to take care of a bunch of baby Pokémon poisoned by a Tentacruel, where their condition looks bleak, and it seems like they could die without treatment...but there's also an episode where a Sharpedo is poisoned, is ill overnight, but has recovered by morning without any treatment being given.

 

Wouldn't all of the OHKO moves be counted as the most deadly moves?

 

Guillotine. I'm sure we can all guess what this move is meant to do, and the fact that it's a one-kit KO move doesn't help matters at all.