I think we equate higher (and often lighter) voices with youth - and often
the central lovers in opera are young.
So, tenors (and sopranos) tend to get pegged for ingenue roles. Lower voices,
with their heavier, darker timbres, tend to be characterized as older, or more
authoritative, or yes, villianous.
Many times tenor roles can be more evil if they're supporting "character" parts -
in Mozart we have Basilio (and perhaps Curzio) and Monastatos. Of course there
are more comic villains than the real thing.
But Canio is certainly more villain than the young lover he probably wishes he
were. And to contrast we get Silvio, the young lover, as a baritone.
And of course there's Busoni's Doktor Faust, where Faust (often the young lover
tenor, in other settings of the story) is made a darker, more severe character as
a baritone, and Mephistopheles is now the tenor. But in that extraordinary
sequence of the 6 flame/underworld spirits in the Prologue, we find out how
Busoni assigned the voice part - the 6 hellish flames go from lowest bass to
highest tenor, in order of how "fast" they are - the slowest/lowest, Gravis,
grumbles slowly starting on a long held low G - by the time Mephistopheles (#6)
beckons with high, sighing, jumpy lines extending to a high C, we discover he is
the fastest spirit - "as fast as human thought."
Some of my favorite moments in opera, though, are where similar voices - or at
least similar voice *ranges* collide in a hero/villain (or other opposing) situation.
Verdi's great male lower-voice duets, whether it be Simone/Fiesco,
Rodrigo/Filippo, Filippo/Inquisitor, or even Falstaff/Ford. AND the vocal "duel"
between Manrico and Di Luna in the Act II Finale of Trovatore, which ends with
both of them singing at each other on the very same pitches. The "Jealousy
Duet" in Threepenny Opera, which was written to be sung by two dueling
sopranos. Likewise the literal vocal duel between the two sopranos in Mozart's
"Impresario." And to bring it back to tenors, the great confrontations between
Jesus and Judas (both tenors) in Jesus Christ Superstar. (I know - most of you
will say it's not an opera. I don't care, lol.)