By that same argument, you probably don't need both Killer Pilum and Killer Spear. Also Pilum have 26 QL is a really odd number. I get giving it even number so that even set of uses but at that point just give it 30 or 20.
Honestly, I don't really see the point of adding *more* crit weapons since each weapon type has some crit weapons and typically giving your PCs more crit is not something a GM wants to do.
Also like... these seem pretty OP?
Killer Flail: Same everything as Steel Mace (C), except 5 less hit for 25 more crit.
Killer Pilum: Same everything as Steel Pilum (C) except 20 extra crit for 4 fewer uses.
Ballistic Dagger: One more WT, 2 more MT, 5 less hit, 10 more crit than similar C rank dagger
Killer Spear: Straight up better than Short Spear in every option
Cleaver: Better everything except hit (tied) than lancereaver, though reaver does swap wta but prolly not enough of an advantage to justify +1 mt, +20 crit
Frost Nova: +3 mt, +5 hit, +25 crit to lose effective vs armored
Conflagration: +3 mt, -5 hit, +25 crit to lose effective vs horseback (even just comparing frost nova and conflagration there's an imbalance)
Sharpshooter: I actually might argue this one is underpowered. It either loses completely to Steel Yari (-25 hit and -5 uses for +10 crit is not a lot) or basically ties Lunar Longbow except trades effective vs armored for 10 crit
Cursed Fire: -2 to 3 wt, +5 to 10 hit and +10 to 20 crit in exchange for -2 to 3 mt is ehh. I think you're discounting how much wt is worth? (Also not really a fan of this cause it ruins the whole gimmick of the line which is they trump a whole bunch of things but lose to even more things where this is trumps like 8 things and loses to 90% of the weapons vs trumps 1/6 of the things, etc)
Nightmare: Simultaneously think it's overpowered and underpowered which might mean it evens out? Stats wise, it's similar everything except +20 crit, but then the effect it has is a lot weaker than Stone (which would prevent actions for a full turn) and forcing opponent to miss their counter attack.
Merciless: This one isn't great but isn't terrible? It loses a bunch of wt and a few mt points and gains a bunch of crit. The only reason this isn't terrible is because usually axe users have a lot of con so the wt isn't as much of an issue, but losing 4 to 7 points of wt and gaining 25 crit in exchange for 2 to 4 points of mt is a big change.
In general, I think you're discounting how much things are worth? In general, a good rule of thumb to have is -/+1 wt=+/-1 mt=+/-5 hit=+/-5 crit buuuut that crit is really powerful. If you look at all the other killer weapons, they're typically either all around worse than the other similar rank weapon or they're about the same but the other weapon has some effect like reversing weapon triangle or effective damage. The only one I'd see as different is the recurve subcat but poison bow sucks so eh.
A good example actually is Killer Crossbow vs Silver Crossbow. They have the same MT, but the Killer Crossbow has +2 wt, -20 hit, and +30 crit. So it follows the stuff I said above.
In general, I don't think the weapon ideas are terrible. I just think they're unbalanced and also that giving more crit to PCs is a bad idea
Edit: I don't know exactly how QL use translates into stuff, but prolly similar +/-5 QL for the above stuff? That one is a bit harder to translate.