Throughout The Vampire Diaries, Damon was often a morally gray character, as he started out as one of the show's villains and tormented Stefan as part of his twisted revenge plot. Damon was impulsive and selfish, resulting in him lashing out ferociously when things didn't go his way.
Whilst he did eventually grow to be a better man and a fan-favorite character, the things he did when he had his humanity turned off were terrible and hard for fans to forget. From the brutal murders he committed, to his complete disregard of others' feelings, Damon was at his worst when he had no humanity in him.
How He Lived In New York
In the flashback episode "Because the Night," Damon recalled the party lifestyle he enjoyed with his humanity off in the 1970s, where he wreaked havoc on the streets of New York. Some of Damon's worst traits were his apathy and impulsiveness, and these were only emphasized when he harbored no humanity.
Throughout the flashbacks, Damon hunted and killed people on sidewalks, manipulated Lexi, and even had a system set up with someone who owned the bar he frequented, whereby he could feed off people as they danced, as long as he compelled them to forget. It was a horrific insight into Damon's life with no humanity, and showcased just how brutally selfish he could be.
Tricking Lexi And Eventually Killing Her
Damon's treatment of Lexi was abysmal, despite her repeated attempts to help him and Stefan throughout the show. In "Because the Night," it was revealed that Lexi had tried to get Damon to turn his humanity back on, despite how badly he had treated her up until this point. Not only did Damon seduce her as part of his trick, he then left her on a rooftop in daylight, and threatened to let her burn if she didn't leave him alone.
As if it couldn't get any worse, he then elected to kill Lexi in "162 Candles," as part of his ploy to get Liz to trust him. For such a sweet and loyal character, Lexi deserved better than Damon's awful treatment and her murder was one of the most impulsive and shocking things Damon did when devoid of humanity.
His Killing Sprees In Season 1
Damon's actions throughout season 1 of The Vampire Diaries were unforgivable and introduced him as an impulsive and dangerous vampire who stopped at nothing to get what he wanted. In the show's very first episode, he was shown brutally murdering a couple who were camping in the woods, and his killings didn't stop there.
In "You're Undead to Me," he killed Zach and went on to mercilessly slaughter a group of teenagers who were hanging out in the cemetery. In the following episode, he killed Vicki and then turned her into a vampire, knowing how dangerous and careless his actions were. Damon killed multiple people throughout the series, but his killings in the first season were among his most vicious because of his lack of humanity.
Manipulating Caroline
Damon's treatment of Caroline in season 1 was truly one of the worst things he did, and the way he talked about her is one of the many things about Damon that haven't aged well. Damon relentlessly used Caroline to satiate his needs, from feeding on her, compelling her to help him, or sleeping with her, he treated her like his own personal puppet.
He did all of this and then compelled her to completely forget all about it. Caroline deserved so much better than to be used by Damon like this, especially as she was only a teenager at the time and was enamored by Damon's charismatic ways. Thankfully, Caroline got her revenge on him in season 2, but that doesn't mean that Damon's manipulation of her wasn't truly disgusting.
Killing Maggie James
Damon's escape from the Augustine society was devastating in itself, but his hell-bent ambition to torture all those who had harmed him made the events even more barbaric as Damon sought revenge. Enzo, during his time in captivity, had developed a relationship of sorts with Maggie, and had even confided to Damon that Maggie was the reason he kept hope.
However, without his humanity on, Damon seemingly disregarded who Maggie was and what she meant to Enzo, instead choosing to follow his own selfish revenge plot. In "Man on Fire," Damon admitted he was the one who killed Maggie, but claimed he didn't know who she was. Whilst this was perhaps a small relief that Damon didn't knowingly kill Enzo's love, it was still a terrible thing he did with his humanity off.
Attacking Bonnie And Caroline
By season 8 of The Vampire Diaries, Damon had developed a great friendship with Bonnie, and he and Caroline had managed to put aside their differences to become friends of sorts. However, with the introduction of the mysterious Siren, and Damon devoid of any humanity, Damon went after his friends.
In "You Decided I Was Worth Saving," Damon turned up whilst Caroline and Bonnie were wedding dress shopping, and ruthlessly killed a shop assistant before throwing Caroline across the room. He then continued to torment Bonnie over losing Enzo, before both women managed to overpower him. Considering what they all had gone through up until this point, it was hard for fans to watch Damon turn against his friends so easily.
Tormenting Stefan
Damon and Stefan treated each other horribly throughout the course of The Vampire Diaries, yet Damon's insistence on tormenting Stefan in the show's earlier seasons was truly formidable. Usually, the terrible things Damon did doubled up to hurt Stefan, as Damon had promised him an "eternity of misery."
For instance, throughout season 1, Damon elected to tease Elena with information about Katherine, in order to get to Stefan and force him into uncomfortable conversations about his past. Damon even threatened to harm Elena in "Friday Night Bites," and proceeded to kill Mr. Tanner in front of Stefan in his efforts to prove to Stefan just how miserable he intended on making his life. Whilst their relationship did improve when they both had their humanity on, Damon was ruthless in his torment of Stefan when he didn't.
Obsession With Zach And His Family
The tragedy of Zach Salvatore is one not easily forgotten by fans and portrayed just how villainous Damon could be. Damon made Zach's life hell whenever he returned to Mystic Falls, through his careless disregard of how he treated others, and how he recklessly killed people with little care how it reflected on the town.
Whilst Zach tried to warm to Damon, it was revealed in the episode "Black Hole Sun," that Damon had returned to town once more in 1994, still lacking in humanity and still vengeful. After an argument with Stefan, Damon killed most of the guests at Zach's dinner party and his pregnant girlfriend, leaving Zach heartbroken. Damon's treatment of Zach was awful, and only got worse as he continued to torment him throughout season 1, eventually leading to him brutally killing Zach in "You're Undead to Me."
Leaving Enzo
Damon and Enzo's imprisonment and torture by the Augustine society was shocking for fans to watch, but the ordeal was only made harder when Damon elected to turn his humanity off and leave his friend to presumably die. "The Cell," followed Damon and Enzo as they hatched a plan to break out of captivity together, only for the plan to go awry.
In a heartbreaking moment, Damon realized that he couldn't free Enzo as the bars of his cell were covered in vervain, and the place was burning around them. Rather than stay and help his friend, Damon chose to turn his humanity off so that he would be able to leave his friend to die without feeling any remorse. It was one of Damon's most selfish acts, and certainly one of the worst things he did with no humanity.
Obsession With Elena
Damon had two main purposes for returning to Mystic Falls in season 1 of The Vampire Diaries: continue to make Stefan's life hell, and get Katherine out of the tomb. Unfortunately for Elena, both of these things inadvertently involved her, and Damon's selfishness overruled any care as to how it would make her feel.
In order to get to Stefan and because of her likeness to Katherine, Damon constantly made advances towards Elena, like when he attempted to compel her into kissing him in "Friday Night Bites," or made her leave town with him in "Bloodlines." Whilst Elena and Damon proved to be soulmates in the end, his initial obsession with Elena was creepy and made her feel uncomfortable and scared around him, which he was definitely aware of and enjoyed.