I'd point out that you could rebrand the quick blade if you don't want to use pain, I mean pain is *really really* good, but a qblade of elec would be pretty good too, and not require necromancy. (elec is about equal to pain brand and 11 necromancy skill)
Here's the list of things that are immune to both pain and elec (FWIW things that are immune to pain are also immune to drain)
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Air elemental
Ancient lich
Angel
Antaeus
Archer statue
Asmodeus
Ball lightning
Battlesphere
Blizzard demon
Bone dragon
Boris
Cacodemon
Charred statue
Cherub
Clay golem
Conjurer statue
Crystal golem
Crystal guardian
Curse skull
Curse toe
Dancing weapon
Demonic crawler
Dispater
Earth elemental
Eldritch tentacle
Electric golem
Elemental wellspring
Ereshkigal
Executioner
Fire elemental
Fire vortex
Firespitter statue
Flying skull
Frost-covered statue
Gargoyle (monster)
Gloorx Vloq
Grand avatar (monster)
Greater mummy
Guardian golem
Hell Sentinel
Ice statue
Ignacio
Insubstantial wisp
Iron devil
Iron elemental
Iron golem
Iron imp
Khufu
Lasher statue
Lightning spire
Lom Lobon
Lorocyproca
Master Blaster
Mennas
Metal gargoyle
Mimic (monster)
Mnoleg
Molten gargoyle
Mummy priest
Murray
Obsidian statue
Orange crystal statue
Orb of fire
Peacekeeper
Pit fiend
Profane servitor
Raiju
Ravenous mimic
Rock worm
Roxanne
Rust devil
Saltling
Seraph
Shock serpent
Silver star
Silver statue
Sixfirhy
Sky beast
Snail statue
Spatial maelstrom
Spatial vortex
Spellforged servitor (monster)
Spooky statue
Stone golem
Sun demon
Tentacled monstrosity
Test spawner
Thrashing horror
Titan
Toenail golem
Twister
Tzitzimitl
Even though the quickblade is slightly faster than the rapier of speed, and thereby gets a slightly better benefit from slaying (assuming you have significant slaying bonus) it's notable that the rapier's non-branded damage exceeds that of the quickblade until you get up into around 70 slaying or so.
Against things that can be damaged with pain, it only takes about 6-7 necromancy to exceed the speed rapier's damage.
Against things that can be damaged with electricity, elec rapier slightly outdamages the speed rapier, but by a pretty tiny margin, not enough that it's worth carrying it around.
In terms of raw damage the draining short sword is inferior to the speed rapier, but draining makes things easier to hex, and reduces the damage they do, so it's not a clear cut comparison.