"Both are tall, skinny, pale, dark-haired disasters with legendary bad attitudes and worse hair.
Both come from grim, working-class British backgrounds (Snape's Cokeworth mill-town hell vs. Sid's council-estate chaos).
Both had massive chips on their shoulders against the "posh" or "golden" crowd (James Potter & the Marauders = the 1970s equivalent of posh art-school posers; the establishment Sid despised).
Both leaned into deliberate ugliness and menace as armor: greasy lank hair + all-black wardrobe + permanent sneer for Snape; spiked hair + torn clothes + swastika T-shirt provocation for Sid.
Both were deeply self-destructive and loyal to the point of tragedy (Snape's double-agent torment; Sid's spiral after Nancy).
And crucially: both became icons of spiteful, anti-heroic cool despite (or because of) being absolute wrecks".