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Clark Kent. He attempts to dictate terms to Oliver about what the Justice League of America is; he tries to kick Oliver out for murdering Lex when he's not even a member of the JLA, and despite the fact that he tried to kill Lex himself; he tells Chloe off for trying to protect him (though maybe he had reason for that - see below) and abandons her at or after Jimmy's funeral. He also trusts the man who put a kryptonite dart in his shoulder and trusts his coworkers so much that he leaves his TIME-TRAVELING RING ON HIS DESK. He experienced the whole Linda Lake thing, and yet he does this. He also loses his faith in humanity because "a human killed Jimmy," even though Davis isn't human and is barely even Kryptonian in the classic sense, not to mention that Jimmy himself actually IS a human.
Chloe Sullivan. She ends up looking wishy-washy; she's shown to have feelings for Clark, Jimmy and Davis; she eventually settles on doing everything for Clark, caring for and wanting to save Davis, and never having left Jimmy.
Another wall-banger in this episode was that Chloe, having brought about the death of her husband and the disappearance of her cousin as a direct result of harboring a serial killer because she had feelings for him — deliberately sabotaged every one of Clark's plans to bring him to justice, claimed that it was for his 'protection' but never explains how. Not only was she not punished at all, and not only did she show no contrition whatsoever, but she allowed Clark to take the blame for the events which she canonically brought about. The premise of this episode included that none of the events in it would have happened if Chloe had died earlier in the season). That was frustrating.
Davis Bloome. Prior to the finale, he's portrayed as not wanting to kill; he only killed to suppress the Superpowered Evil Side that would kill thousands more. He already exhausted the option of suicide. After being separated from Doomsday, he murders Jimmy in a jealous rage because Chloe lied to him. The actor himself had a problem with the scene, and told the writers so. Doomsday doesn't get away scot-free; the wimpy fight ends with Clark burying Doomsday under the earth, despite the death anvils Clark had been getting and Doomsday's reputation.
Made even worse by Davis and Doomsday being separated at the time. Davis' entire arc was about giving Doomsday a human side to make killing him not so cut-and-dry; any other show would have his Moral Event Horizon be the lynchpin that makes sealing Davis to stop Doomsday the right moral choice but Smallville decided to both Take a Third Option with regards to this and have Davis Jumping Off the Slippery Slope, making said 3rd option not only utterly meaningless but directly responsible for Jimmy's death.
Jimmy Olsen. Or rather, Henry Olsen, as a retcon surpassing even Veritas rears its head. After being on drugs for so long that he steals money in his most recent appearance, he's suddenly fine and well-adjusted. Not only that, but despite his need to steal money and his reconciliation with Chloe not coming until later, he also has enough money, somehow, to buy and pay bills on a beautiful gigantic loft that is somewhat anviliciously referred to as the Watchtower. And he is first killed by Davis and then revealed to be the older brother of the Jimmy Olsen fans recognize in an effort to realign the show with the mythos they've ignored for seven seasons. Never mind that we've never seen this younger brother - his own ex-wife hasn't met him prior to his funeral - and his horrible home life (also shoehorned in last-minute) should have suggested that, at least when he was sober, he wouldn't allow his younger brother to be left alone with his dad.
Oliver Queen and the Justice League of America. Apparently, the JLA is a-okay with murder; they accept Oliver's murder of Lex without incident. Then they help Oliver trick Clark into coming to rescue Chloe and shoot him with Kryptonite. Then they assumed that they could somehow kill Doomsday with a bow and arrow, super-speed, and a high-pitched scream. Further, Dinah, under the username "Black Canary," sends emails and an audio clip of her voice to Clark at his work computer, which is known to be monitored; it's not immediately apparent what's wrong here, but when you remember Dinah's and Clark's professions (famous radio show personality and newspaper reporter, respectively)... While Tess may have known Dinah's identity, others probably did not; and the clip was somewhat loud.
How about the fact taht their entire "kill Doomsday" plan wouldn't have worked and they know this? Oliver is fully aware that Doomsday can come back from the dead. He's the one who buried him in the first place. Yet he still falls back on "I'm gonna shoot him in the head" and seems to think it will work.
Lois Lane. While Lois hasn't always been portrayed as exceptionally intelligent, she drops to a new low by chewing Clark out for not searching for Chloe when she herself is also sitting at work. After seeing Clark writing a letter (which she sees for at least an instant), she is in no way suspicious of Clark when he disappears and the Retter Business Bureau calls her to publish his letter. She then puts on a time-traveling ring for no good reason, without knowing to whom it belongs or what it does.
Tess Mercer. Even though she knows that both Clark and Davis are Kryptonian and have a motive to steal her orb, she questions Lois about the break-in. This could be forgiven because she's seen Lois (or someone she believed to be Lois, but who was Faora) kill a man and use super strength and perhaps speed. But Lois has no motive for stealing the orb, and she certainly displays no great strength or speed in her fight against Tess. Tess also ignores her minion's report that the vault containing the orb was blasted open from the inside.
Further, the aforementioned fight between Clark and Doomsday that had been teased for the entire season and heralded as the biggest challenge Clark would face by both Jor-El and Rokk lasted under two minutes and was released ahead of time in a Director's Cut. The whole "buried deep underground" thing is obviously a reference to the arrival of Doomsday in the comics, with a few big problems. In the comics, Doomsday has just been killed before being buried, making him at his weakest, and there is no sunlight to recharge him. He is also bound by cables Made of Indestructium. There is absolutely no reason why in Smallville Doomsday can't just crawl back out. It's no surprise that Smallville was moved to the Friday Night Death Slot for Season 9.
You heard us. Two minutes. In the comics, Superman and Doomsday's clash tore up Metropolis. It leveled the streets, demolished buildings, and killed them both. Here...it was under two minutes.
In the Grand Finale, Lex has been brought back to life, still has all of his memories and knows Clark's secret. In his final scene, he fatally stabs Tess, who uses some sort of toxin from Summerholt to erase ALL of his memories "up to this moment". The show spent its first seven seasons building and ripping apart his relationship with Clark, which would turn them into bitter enemies, and in the space of 30 seconds, they completely and utterly destroy everything they had worked towards. Now, Lex probably doesn't know who he himself is, can't remember Clark or his secret, and somehow gets elected President of the United States within 7 years! It's just as bad a wallbanger as the Henry James Olsen retcon.
What, nothing about the final battle? They spent all of season 10 building up to some epic battle between Clark and Darkseid. In the Grand Finale, what actually happens? Darkseid (while still possessing Lionel Luthor, so we never even get a good look at his true form) hits him with a telekinetic blast, then Clark gets an overly-long series of flashbacks, finally learns how to fly, and smashes through him. That's it. Especially a letdown after Justice League Unlimited.
Martha hiding the Key from Clark and Jonathan because she was scared that Clark might leave them. Okay, before, I found her character to be kind of iffy, since, while they had good reasons to keep Clark's abilities a secret, it's obvious that before his abilities, she was so desperate to have a baby that she didn't care about having a legitimate reason. But this? I'm sorry, but this crosses the line completely. It's so many levels of selfish that I was happy when she collapsed. Mama Bear or not, this act was outrageous.
Chloe's entire character in "Truth". She abuses the hell out of her powers. She makes a guy tell her who he likes, even though she should be sensitive regarding this due to how she was with Clark, then she just laughs at his embarrassment when he is forced to out himself as a homosexual. Then, she tries to use her powers to force Clark to reveal his secret, even though she had already learned to respect his privacy (they even lampshade it!). And of course, she acts smug the whole way through. Then, when she accidentally forces Pete to reveal that he has feelings for her (while trying to make him reveal Clark's secret, so abusing their friendship), she seems slightly remorseful. Yes, when it involves her friends, not when she made a guy out himself as gay. Of course, when she loses her powers, she is easily forgiven. Yep, even though she ruined two lives (possibly three, given how homophobic many high schoolers are), and she herself states that the powers likely didn't affect her personality.