As for the marriage of 17-year old Melissa Steiner and 40+ year old Hanse Davion, remember here we're talking about nobility, and many of the people on these forums who were here before BattleTech moved have played Crusader Kings 2 (in fact playing Crusader Kings 2 is a good way to get into the mindset of a Successor Lord, there's a non-zero chance you will do muuuuch worse than most of the Successor Lords). Marriage alliances are just sort of a thing that nobility do, regardless of age. To his credit Hanse Davion gave every effort to make it work and it grew out of a marriage of political opportunity into a marriage of genuine love.
In terms of mustache twirling villains, Stefan Amaris (who had a rocking fu-manchu) Mad Max, and Katherine "Katrina" Steiner-Davion take the cake (and the second worst of them by far is Katherine, she literally murdered her own mother and tried to frame her brother, and tried to murder her lover). In fact the Lyrans were, through unity with the FedSuns under the FedCom and Hanse and Victor, were setting aside their military issue of social generals before Katherine undid them all because conveniently all the people who actually supported her were the cronies who had been chafing under the fact that they couldn't be social generals anymore.
Myndo Waterly, future Primus of ComStar, is up there too, as she stabbed the Inner Sphere in the back until it was obvious the Clans were coming for ComStar, too (and then tried to stab everyone in the back at once which backfires horribly and hilariously). Jerome Blake probably also qualifies, OLD lore indicated that his successor murdered him and went maniacle with the religious aspects, but NEW lore has indicated it was entirely Blake's plan and his successor was crushed but dutifuly went along with it (opening of the 2SW sourcebook for people wondering).
If you're seeking the overarching villain up until Tukayiid ComStar is it, then the Clans (more specifically the Crusaders) take over, then Katherine Steiner-Davion, then the Word of Blake (ComStar 2: Electric Boogaloo, We're Gonna Throw A Tantrum Because [ MOD Edit: Please keep in mind this forum is closer to E for everyone rather then the PG-13 of the old forum in regards to language] YOU EAT NUKES AND DIE (also the setting is getting too advanced so a bit of a reset button!)). I don't know much about the lore of the Dark Ages or beyond, too. Of the Successor States it probably goes, in ascending order of Goodness, Liao < Kurita < Davion < Steiner < Marik at the time the game kicks off, though as stated it's more the people rather than the nation themselves.