So one of the quirks of the Nightmare difficulty is that monsters respawn.
This means you can actually achieve more than 100% kills for a given level, making the distinction of 100% kills pretty pointless. Notably, enemies resurrected by Archviles (on all difficulty modes) also count towards your total kills and can similarly take you over the level's official enemy count to produce more than 100% of the kills.
This is why you sometimes see Ultra-Violence (or any non-Nightmare difficulty) speedruns with the tag 'fast monsters' since that's another really notable change of Nightmare. The fast monsters setting when combined with the Ultra-violence difficulty effectively creates a difficulty between Ultra-Violence and Nightmare that still lets a runner achieve 100% kills.
Here's some Doom speedrunning categories you might see in relation to all this (The names vary by site, but these are some common ones):
- UV Speed (Ultra-Violence)
- UV Max (Ultra-Violence, 100% Kills, 100% Secrets)
- UV Fast (Ultra-Violence, 100% Kills, 100% Secrets, Fast Monsters)
- NM Speed (Nightmare)
- NM100S (Nightmare, 100% Secrets)
For anyone interested in the details of the difficulty modes, I'd suggest you check this video. The channel also has plenty of others that can help you understand the gritty details of how the DOOM engine and the games work.