-backthrow only kills on stages with walkoff edges or at high damages.
-Dsmash only hits ungurading enemies on the ground ( mean it has a really low hitbox) AND has terrible startup-lag, cool-down lag and priority.
-Fsmash is slow and normally easily avoided.
-bair is pretty solid... not amazing, but it's about all sonic's reliably got
- Dair has slightly over average knockback but is easily punishable and not often practical.
- Fair only has good knockback at the end of the attack, and can be DId in practically ANY direction except down. besides, you'll be using this move primarily to rack up damage... you'll only kill with it at high damages or near a screen edge
-some other moves can kill (nair, Utilt, Uair, homing attack) but pretty much only at VERY high damages and when they have NO move decay to that point.
-his edgeguaring game is pretty good, as is his pressuring ability, but it's not enough to make up for his poor priority for all of his kill moves and his general lack of strength.
-so, in recap: sonic has only two or three real kill moves:
#1 - Bair
#2 - Fsmash
#3 - Dsmash
Neither his Fsmash, nor his Dsmash really count as a RELAIBLE kill move, but both are pretty good, and together they at least constitute 1 kill move. Really, once you play against decent players, you'll see how much trouble he has killing