Sonic has literally SO many kill moves, i honestly just think of him at this point as pikachu with actual ways to kill besides smash attacks.
Off the sides: Nair (Sweetspot it, always) - Bair (same as nair but not as imporant) - Side B sweetspot kick (Great ledge cancelled) - Fair (The 2nd and 3rd hitbox on the very outside hits harder then you think) - Dsmash - Fsmash
Off the top: Uair - Utilt (For high percents like 130+) - Neutral B (this is eh, only floaties will die to this and only at extemely high percents)
Gimping tools: Fair (Meteor) - Dair (Spike) - Nair (Sex kick) - Neutral B - Side B hop
so yeah lots and lots and lots of ways for sonic to get the kill, the way i play sonic mostly is very very simple: I aim from the first second of the game to get them into a position where i can Fair/Dair them off stage depending on the percents. At lower percents go for the fair because dair spikes will literally get you punished untill 35+ where they send into tumble. I look to build percent from edgeguarding if i cant outright kill them through the gimp then at around 60+ you can start looking for more solid kill options like Bair, Nair, side-b kick, and uair for floaties (Usually uair combos to itself so the sooner you can start doing this to floaties and just following there trajectory down while threatening with more uairs the better considering they do 17%, 13% on the second hit which is what you wanna land if you cant get both). Once they get to the 100+ mark is where you can just look for one good sweetspotted nair, bair, side-b kick, dsmash, fsmash, or uair on floaties and secure the kill for sure
During all percents though everytime you have Fair meteor or Dair spike as a safe option off stage you should be going for it, eventually you just learn when not too (Im still learning this lol)
As far as set ups for all of these moves go:
Dtilt>Nair, Bair, Uair, Fsmash
Utilt>Nair, Fair, Dair
Uthrow>Nair, Bair, Fair, Uair, Dair
Ftilt>Nair, Fair, Dair
Those are just a few examples of 2 hit strings i use to get the kill. Alot of moves arent even listed in there because most of the times youre not gonna get a nice clean set up like those are just gotta be able to work with what you got on the fly playing sonic.
All in all, sonic is one of very very very few characters that can kill from 0% and can continue to kill at every percent beyond that, its nice not getting Marthed