From the beginning of Grey's Anatomy, Meredith and Derek proved to have a great love story, and the chemistry they shared was palpable from the moment they met. After all they went through, they were ultimately one of the best couples on the show, even after Derek's untimely death.
Despite this, Meredith and Derek did many questionable things to each other, when they were together or not. Both of them could act selfishly at times, and this often resulted in each of them doing wrong and causing harm to the other in ways that weren't easily forgotten.
Derek Didn't Tell Meredith He Was Married
Addison's introduction to Grey's Anatomy was nothing short of iconic in the episode "Who's Zoomin' Who?" and her entrance disrupted Meredith and Derek's relationship massively. Throughout season 1, while beginning a relationship with Meredith, Derek neglected to tell her that he had a wife. Despite the fact that Addison had cheated on Derek, which caused to him leave New York, they were still husband and wife.
Derek knew that he was quickly developing feelings for Meredith and that she was reciprocating those feelings, yet never told her about Addison, which would have definitely altered the trajectory of their relationship. It was a sly move on Derek's part, and the shock of Addison's arrival made the news ten times harder than it would have been had he just been truthful from the start.
Meredith Dated Finn
Season 2 was tumultuous for Derek and Meredith, and followed them as they officially broke up and Derek tried to get his marriage with Addison back on track. Whilse Meredith and Derek were apart, their relationship was still very dramatic, particularly when Meredith started up a romance with the vet that their dog, Doc, went to.
Her relationship with Finn was one of Meredith's best romances on Grey's Anatomy, but that didn't mean that it wasn't an unfair thing for her to do, considering that Derek frequently had to see them together. Meredith and Finn's relationship only caused more strain on Derek and Meredith's ongoing attempts at friendship, and also created more tension for Derek with Addison.
Derek Criticized Meredith For Being Single
Derek often acted impulsively when he was jealous, and this was particularly true when it came to Meredith. Having already broken up, Meredith and Derek tried to become friends with each other, with Derek even prompting Meredith to tell him why there was tension with her friends as she had slept with George.
The news of Meredith's tryst with George, coupled with the revelation that Meredith was now dating Finn, caused Derek to act completely irrationally, which resulted in a massive argument between the two. Derek crudely suggested that Alex would be the next person Meredith would sleep with, as he too slept with a lot of other people. It was a heated moment, and showcased just how unfairly Derek treated her when he was riddled with jealousy.
Meredith Shut Derek Out
Meredith was notorious for shutting other people out when things got tough in her personal life, and this was no different when it came to Derek. Despite the two seeming like they had worked through their issues, Meredith continued to push him away throughout season 3.
Most notably, Meredith refused Derek's kindness when she was struggling after her Mother's harsh words in "Wishin' and Hopin'," and in the narrative arc following, even when she was so obviously suffering. Meredith's penchant for wallowing in her darkness alone often alienated her from her friends, and the same happened with Derek. Despite his repeated attempts to support Meredith, she pushed him away and thus, the progress they had made in trusting each other again became fractured.
Derek Gave Up On Meredith
Despite fighting relentlessly to be with Meredith throughout season 3, Derek seemed to lose all hope in their relationship, and ultimately gave up on her in her time of need. Her relationship with her father was one of the biggest obstacles Meredith faced on Grey's Anatomy, and Susan's death in season 3 only made it more complicated and affected her and Derek's relationship massively.
While Meredith was understandably distraught, she sought to lean on her friends rather than Derek, which left him feeling rejected by her, and that she didn't trust him. With this, Derek selfishly made Meredith's grief about their relationship, and took her personal troubles as a concrete sign that she didn't trust him. This ultimately led to him giving her an ultimatum on whether she actually wanted to be with him in "Didn't We Almost Have It All?" when he should have been more understanding as to what she was going through.
Meredith Took Zola
Meredith's tampering of Derek's clinical trial left many ramifications for her to deal with, but none more significant than the strain it put on her and Derek's relationship and thus, their adoption of Zola. The episodes "Free Falling," and "She's Gone," detailed the fallout of Meredith's actions, which led to Meredith taking drastic measures to keep Zola in her care.
When Zola's adoption counselor visited the hospital and sensed the fragile state of Derek and Meredith's relationship, Meredith panicked and impulsively took Zola and hid, without telling anyone. While Meredith, with Cristina and Alex's help, managed to cover up her rash mistake, it was wrong of Meredith to not tell Derek and cause him unnecessary worry, even if they were fighting.
Derek Threw The Engagement Ring
By season 5 of Grey's Anatomy, Meredith and Derek's relationship was finally on track as the two had worked through their problems and built up a strong trust in each other. However, Derek nearly ruined it all in "I Will Follow You Into The Dark," when he carelessly threw the engagement ring he had bought for Meredith into the woods after Meredith tried to comfort him.
Derek was understandably struggling at the time, as a surgery he had performed went wrong and his patient tragically died. However, considering all that he and Meredith had been through and all the times that he resented Meredith for pushing him away, it was wrong of him to do the same to her. Throwing the engagement ring while she watched was a rash and mean thing for him to do no matter what he was going through.
Derek Went Back On His Promise
Meredith and Derek were arguably one of the best couples on Grey's Anatomy, but by season 11 their relationship had become tense and the cracks were beginning to show. Having promised Meredith to take a step back in his career so that she could focus on hers, Derek became resentful and withdrawn, and pressured Meredith to do something worthy with the time he had supposedly given her.
However, as Derek's attitude worsened, Meredith began to lose her patience with him, which resulted in a bitter argument between the two in "Risk." Derek's behavior both before and during the fight was selfish and, rather than give Meredith the time she needed, caused her more stress, eventually leading to Meredith telling him to go to DC anyway.
Meredith Messed With The Alzheimer's Trial
The events at the end of season 7 were cataclysmic for Derek and Meredith's relationship, after Meredith tampered with the clinical trial Derek had started to help people who suffered with Alzheimer's, initially inspired by her Mother. While trying to ensure Adele was given the drug and not the placebo, Meredith caused multiple problems with dire consequences.
Despite her good intentions, Meredith ultimately ruined the validity of the trial, got Derek blacklisted from further research, and broke Derek's trust in her. Her actions were impulsive, and caused a massive divide between her and Derek that went on for many episodes, and even threatened their adoption of Zola.
Derek Kissed Someone Else
His unfaithfulness to Meredith in season 11 was undoubtedly one of the worst things Derek ever did on Grey's Anatomy. Before he had even gone to DC, Meredith and Derek had argued about the fate of their relationship, as Meredith didn't want to just be following around after Derek as her Father had done with Ellis. In this same argument, she also expressed his affinity for making people like him, highlighting how he had charmed her when she was an intern, and he was her superior.
Low and behold, Derek seemed to do the same in DC with one of his research fellows, Renee, who instantly took a liking to Derek's charismatic ways. Perhaps not unsurprisingly, Derek liked the attention he received from her, which eventually led to an awkward kiss. While Derek did stop the kiss, it was still one of the most awful things he did to Meredith, especially when their relationship was already in a critical state.