Elena's character was a divisive one in The Vampire Diaries fandom. Some viewers loved the sweet protagonist while others thought she was too annoying and helpless in most situations. Either way, Elena Gilbert was the crux of The Vampire Diaries, and the recently ended TVD universe began with her, ran on her story, and changed based on her decisions.
For a seventeen-year-old, Elena had been through too much tragedy, but some of her decision-making in the show was very poor. She made enemies and allies out of the wrong people, hurt her loved ones, and indulged in activities that would only harm her later. Elena's arc would have looked very different if she had had some foresight and avoided these bad decisions.
Refrained From Telling Jenna About Vampires
As a character, Elena was young and confused. Her losses, combined with her fascination with the supernatural led her to get involved with vampires, and while there was nothing wrong with that, she should have let her Aunt Jenna in on her secret for everyone's safety.
Vampires were not reliable or trustworthy, especially since they were all out to get Elena. If she had told Jenna about Klaus and all the supernatural drama, her aunt would have been more careful when she met new people, and probably would not have become such an easy victim of Klaus.
Made An Enemy Of Rebekah
Elena changed a lot during her arc on TVD, and sometimes her mean and spiteful side would take over. Knowing Rebekah's weakness and naivete regarding men and her missed school experience, Elena took a chance and exploited her, which was not a wise decision to make.
When she could have made an Original ally, she decided to stab her in the back and make a mortal enemy of her instead. Her actions led to Rebekah taking away the one thing that Elena treasured the most: her humanity.
Told Stefan To Save Matt First
Rankled by Elena's betrayal of her, Rebekah decided to be her most villainous and killed Elena in a car crash off the very bridge her parents died. Even then, Elena wanted to control the situation which led her down a dark path.
Stefan struggled to save her first, but even then Elena made the terrible decision of making Stefan save Matt before her. Because of her self-sacrificing thought, she died with vampire blood in her system and woke up as a newbie vamp, which was something she never wanted for herself.
Turned Off Her Humanity
Turning off her humanity was the worst thing Elena did, and this sole decision led to a lot of cruelty and meaningless death at her own hands. She did not know how to cope with the grief of losing Jeremy, which was why she listened to Damon and flipped the switch.
Innocent bystanders were killed, both the Salvatores were hurt, and best friends were attacked and ill-treated, which was not something Elena would do otherwise. A lot of regrettable actions followed her decision to flip her humanity switch.
Trusted Elijah To Kill Klaus
Despite everyone's better judgment, Elena placed her trust in the "noble" Elijah, who promised her that he would finally slaughter Klaus once he was weakened while breaking his Hybrid curse. It was foolish of Elena to make a pact with Klaus' own brother for his death, especially since she knew nothing about their family.
As predicted by the other more experienced vampires, Elijah felt love and pity for his Hybrid brother right at the last moment and betrayed Elena to save him instead.
Chose Damon Over Stefan
Delena shippers would disagree vehemently, but Elena's final choice between the Salvatore brothers was not a well thought out one. She got swept up by Damon's charms and forgot all the atrocities he had committed against her and her friends in the past.
Stefan always respected her choices, and gave the same courtesy to her family and friends. He was, objectively, the better person and partner to be with, and Elena should have made the more certain choice of her first love.
Compelled Jeremy Several Times
For someone who believed in giving people the freedom to choose, she took it away from her brother too often. Elena kept requesting her vampire friends to compel Jeremy to forget things that were close to his heart, like his first girlfriend Vicki.
Elena even managed to compel Jeremy to move out of Mystic Falls, which ultimately turned out to be a terrible call, because Kol found him there, and he was dangerously unprotected.
Took Witch Drugs After Damon Died
Clearly, Elena couldn't process grief, and it was hard to blame her for it. She had seen too much loss too early, which was why she took impulsive decisions to soothe herself and make things better for herself.
So, when she lost Damon, her sadness spiraled out of control. She started blackmailing Luke for witchy drugs that would make her hallucinate Damon, but they also had awful side effects. Her worse instincts would come to the fore, and that resulted in innocent people in Mystic Falls getting hurt.
Used Bonnie As A Witch For Hire
Elena called Bonnie her best friend, but she never treated her like one. When Bonnie died, Elena was so caught up in her newly minted relationship with Damon that she did not even realize that her best friend was not on vacation, but dead.
She allowed Bonnie to be used for her own ends. The Salvatores turned her mother into a vampire and forced her to her physical limits to perform spells for them all the time, and Elena stood by and watched as they treated Bonnie badly.
Let Her Guard Down Around Katherine
Katherine was the most evil villain of TVD, and she had always had it out for Elena, so it was strange that Elena showed her compassion in her dying moments. Katherine was dying because Elena shoved the cure down her mouth, so it was a bit obvious that Katherine would take revenge.
Elena got too close to her Doppelgänger, who body snatched her extremely easily because of Elena's trusting nature. Elena should have kept her guard up, or stayed away from her completely.