In my personal opinion,
powerscaling intelligence/evilness is completely subjective and also OP didn't even list a top 10 to debate over, but trying to think of an example specifically under the OP's rules, I have to say that Mega Malamar (specifically from Pokemon: Legends Z-A's prerelease marketing campaign) is one of the more evil fictional characters in recent memory with no actual stated motive in its story to date, at least in terms of actual impact it had on the world and also how it annoys me personally. I mostly say this because Mega Malamar had a very notable real world impact based on a shaky premise that I heavily disagreed with (you could argue Three Houses can also rank up there but those characters at least have intended motives enough for people to side with/argue over to no end and also I am not really invested in the non-gameplay discourse).
In September 2025 (just a month from Z-A's release), the Pokemon Company cooked up a marketing campaign called "My Friend Malamar" (
detailed on Bulbapedia here), which somehow took the internet by storm, seeming to actually influence the online masses and forced them (in-character) into thinking that (base) Malamar was their friend, leading up to a set of videos in the format of in-universe news footage revealing that Mega Malamar was rewriting peoples' personalities in Lumiose City to become artificial followers who can only think about Malamar at any given moment, paralleling the real life Malamar marketing campaign. Again, there is no explained motive, but the
Legends Z-A website entries claim that Mega Malamar is intelligent, haughty, and notably difficult to interact with, viewing all others as pawns in its world and even hypnotizing its own trainer if it disagrees with an order. This evokes a lot of supervillain behaviour, but for all its boasting it doesn't really display any feats of intelligence to me, more just brute force and overconfidence while literally dismissing the feelings of others.
By all means, the ZA Marketing Malamar is made out to be the pure essence of power-hungry evil in the Pokemon world with nothing from its perspective to sympathize with, and yet people gladly join in its fun and games because I suppose being evil and mind controlled to have no personality is simple enough to play along with, and/or perhaps just to follow along and fit in while everyone else you know seems to be involved in the latest online trend, preying on the fear of missing out. Of all the contentious additions in ZA, Mega Malamar represents the lowest of the low of industry plants to me purely because of all of this unwarranted attention.
At the time I was a major detractor of Mega Malamar on the forums even before the game's release, pointing out that
mega evolution had really nothing to do with the plot which could have been performed by base Malamar (and which the real-world marketing highlighted with the fact that everyone was playing along with a trait that base Malamar already had), in contrast to the previous in-universe mega campaigns (Mega Victreebel, Hawlucha) highlighting the bond between trainer and Pokemon to reach new heights or make up for established shortcomings of the base form like showing off in battle. Meanwhile, the concept of Mega Malamar itself requires you to suspend your disbelief and assume that base Malamar, described as having the most hypnotic powers of all Pokemon, is incapable of "rewriting personalities" like Mega Malamar does, but also simultaneously believe that these powers do exist in the world and Mega Malamar is just more capable of them. Conversely if you can believe that Mega Malamar is capable of pulling off the actions in the Z-A marketing campaign in-universe, there's no reason not to believe that base Malamar is incapable of the same thing.
Building on an analogy I made at the time, it's like if they made a marketing campaign all about Machamp having 4 arms and why having 4 arms is so cool, and then they reveal Mega Machamp still having 4 arms but also being vaguely "smarter" and also being able to punch 1000 times in 2 seconds (which is a fact already in base Machamp's pokedex entries so you have to believe that's still impressive or that base Machamp is less capable). And then in the actual games, Machamp having 4 arms doesn't really affect its battle gameplay at all, which is already true but at least you have to respect its beefy Attack stat and unique Dynamic Punches. Unfortunately the gameplay-story inconsistencies just get even worse for Mega Malamar after all the artificial buildup.
When Legends Z-A released, it would turn out that Mega Malamar had notably bad stat allocation which didn't particularly benefit from Z-A's gameplay mechanics, in which it had no ability to take advantage of its former gimmicks, and wasn't represented in a rogue boss battle with unique enemy attack patterns. Even Hypnosis is still just a status move and in Z-A it doesn't even put you to sleep, just makes you drowsy. In fact, Mega Malamar has much less relevance to the plot of Legends Z-A than you would expect (a major character uses one in battle just to reference XY and is not mentioned otherwise), not even being the intended star of any side missions (contrary to the marketing campaign's proposed plot of being a villain causing chaos in the city), and the player can't even look forward to using it in the base game campaign because it has one of the worst availabilities of the newly introduced megas, with its mega stone only obtainable at the very tail end of the main story (and not even given to you for a dramatic story reason like the other endgame mega stones, it's just added to the optional shop with no fanfare in the middle of the endgame story).
To conclude, imagine little Timmy getting Legends Z-A for Halloween or Christmas, maybe with a fresh Switch 2 to boot, hearing about Malamar being such a popular mon online for a few weeks and building up anticipation throughout the entire story and suite of boss battles, only to end up disappointed that Mega Malamar is actually barely in the game. Now I'd say that's a pretty evil thing to do to a kid for no real reason or gain.
tl;dr: Mega Malamar ends up as a fraud in both story and gameplay, especially after its whole successful marketing campaign doubling down on it being a pure evil villain, and yet it actually seemed to influence real life people into believing its contrarian lore with no substance. There may be more terrible characters within fiction, but the uniquely real-life combination of being an industry plant featuring in false advertising (relative to its role in the actual Z-A game) is one of the more evil things a fictional character can successfully cause (while also being relatively in-character and as part of officially promoted media), in my opinion.