Killua speed blitz Sasuke and then punches his head off.
Killua is massively hypersonic. Ever heard of his ability called God Speed. He rips Sasuke's head off before he even moves.
Proof that he is massively hypersonic? The cheetah chimera ant was considered to have incredible speed (weak attacks though) and he was at best mach 15 for his bullet timing feats, but likely less than that, that's just to give you a scale of HxH speed before you start saying characters are MHS. It would be interesting if you could prove it though
Isaac netero only trained for faster than the speed of sound punches. And that was when he was young he has grown way stronger since then. And he didn't even train for it he became great at martial arts and decided to spend his life praying and throwing 10000 punches to the gods every day. Him punching at the speed of sound became a side effect of that.
Either way killua's reaction speed,assasination techniques, physical durability, strength and hand to hand skill puts him above sasuke. Killua in the hunter exam removed a guys heart as soon as the fight began in an instant and he wasn't even bloodlusted. Sasuke will not survive because killua won't give him the chance to put up susano'o. In the rare case he does sasuke can't tag killua in god speed mode and killua isn't ashamed to retreat for a moment in a fight. Sasuke will reveal susano'o before killua even has to use a technique which will put sasuke at a disadvantage especially because it doesn't last forever.
Killua doesn't require constant god speed to match up with sasuke. Him going god speed puts him over sasuke completely. Susano'o will run out way before killua's stamina depletes. And all this is going based on if killua allows the fight with sasuke to drag on.
Him at beginning of series.