Throughout her MCU journey, Nebula has gone from a minion of Thanos to fending for herself, to a fully-fledged Guardian of The Galaxy. When she was first introduced, she had a much darker role and as time went on, she was used to alleviate some dramatic moments with comedy.
Her whole life has been spent needing to please Thanos which has led her to carry out some certainly evil tasks. Even after leaving her father, her actions were questionable and there is a lot that fans may still not know about her, but it is a life full of torture and hatred that led her to these actions.
Tried To Take On Thanos Alone
Nebula's actions here are totally understandable. Thanos has tortured her and ripped her apart, scarring her for life mentally and physically. There are two occasions where she tries to overpower Thanos, once after the events of Guardians of The Galaxy: Vol. 2 but before Avengers: Infinity War, where she tried to sneak on board Thanos' ship, and another when she hit him with her ship on Titan and proceeded to attack him by herself.
These actions are not the "worst" in terms of evil, you could even argue it was sheer bravery, but it is "worst" in terms of foolish. There is very little chance that on her own, even with her enhanced parts, Nebula would be able to overpower the mad Titan.
Threw A Xandarian Out Their Ship
In Guardians of The Galaxy, Nebula cuts off her own hand to escape the battle on Xandar and fend for herself after she refuses to help Gamora and the other Guardians defeat Ronan. She falls from the ship and lands on the front of a Xandarian spacecraft where she picks the inhabitant up and throws him to the ground below from the height of your average airplane.
In terms of evil things MCU characters have done, this isn't that high up, but for Nebula, throwing someone thousands of feet to their death has to be up there.
Tried To Kill Gamora
In the first movie, the team is on Nowhere after they meet with The Collector. Ronan and Nebula are tasked with going to Nowhere and retrieving the orb and a chase ensues involving Rocket, Gamora, Nebula, and Quill.
Gamora, realizing she's trapped, heads to space to try and evade Nebula who fires a destructive blast to Gamora's pod, knowing full well she was in there. Instead of injuring the pod and retrieving the orb, she blasts Gamora's ship into pieces and leaves her to die floating in the void of space.
Tried To Kill Gamora Again
Just before Nebula escapes the battle on Xandar, she and Gamora engage in a fight on Ronan's ship where they both try to kill each other, Nebula obviously on Ronan's side trying to prevent the Guardians getting to the Power stone.
Nebula almost manages to kill Gamora by trying to stab her with an electric sword but the sword throws electricity through Gamora who is holding onto it. Gamora screams out in pain multiple times. Even though it's revealed in Guardians of The Galaxy: Vol. 2 that Nebula just wanted a sister in Gamora, she certainly doesn't seem to be holding back.
Sided With The Ravagers
In Guardians of The Galaxy: Vol. 2, Nebula sides with the Ravagers, who have turned on Yondu, in return for the bounty of units attached to Yondu and the other Guardians. As part of this plan, Nebula shoots Rocket, knocking him out while also shooting Yondu's Finn off the top of his head.
Fans also can't ignore the fact that she betrayed Baby Groot's trust by saying she would help fight the Ravagers if he let her out.
Tried To Kill Gamora A Third Time
In Vol. 2, Nebula finds Gamora on Ego's planet and tries to kill her in numerous ways. She first fires the ship's guns on her, then tries to hit her with her ship, and then tries to strangle her. All of this takes place within a 4-minute fight sequence.
She almost succeeds but stops just in time and the two discuss their differences as sisters and daughters of Thanos. Even though they reconcile their differences, Nebula still did all of the above after Gamora spared her life. Thankfully, this was the last time she tried to kill Gamora (until 2014 almost shot her in the third act of Endgame). By now you might start to wonder if they will ever truly be family.
Allowed The Murder Of Yondu's Men
After Nebula sides with the Ravagers, a full-scale mutiny takes place where Taserface ejects Yondu's men into the dark void of space with Yondu watching the life get sucked out of them as they float into the void.
They could have been imprisoned, even murdered quickly but Yondu's men being ejected into space is a very vicious death and one that only the cruelest of the cruel would let happen. As the daughter of the most powerful being in the galaxy, you would think she could have campaigned for a different murder method if she wanted to.
Joined Ronan
In the first movie, Thanos sends Nebula to Ronan's ship to aid him in retrieving the Infinity Stone from the Guardians. However, once Nebula realizes that Ronan, with the stone, could defeat Thanos, Nebula agrees to help Ronan destroy a thousand planets if he kills Thanos.
On the scale of good to evil, it's pretty clear that joining a bloodthirsty Kree to destroy thousands of planets and then turning on your father is up towards the evil end.
Chose Thanos Over Her Future Self
In Avengers: Endgame, 2014 Nebula meets 2023 Nebula who was freed by 2014 Gamora once she learns about her future. The two try to convince the 2014 Nebula to join them and help defeat their father. You would think a basic rule of life is that if your future self came to you and gave you advice, you should listen to it.
But 2014 Nebula goes to fire a shot at Gamora to kill her (again) but is shot first by her 2023 self. Trying to kill your sister and siding with your arguably insane Titan of a father while ignoring your own advice from the future is definitely one of the worst things Nebula has done.
Brought Thanos To 2023
After 2023 Nebula got caught in Avengers: Endgame, the 2014 Nebula poses as her and returns to 2023 with the rest of the Avengers after the time-heist. Once back in 2023, Nebula reprograms the time machine to bring 2014 Thanos to 2023 to retrieve the Infinity Stones and carry out his plan.
Doing so destroyed the Avengers compound and caused the deaths of countless warriors from all over the universe. Not to mention if she hadn't done this, by extension, Tony Stark would have never died.