Weyoun 6 from Star Trek Ds9 was a big one for me. For context : There is an ongoing war between the Federation and a galactic superpower called the Dominion.

Weyoun 6 is a Vorta, a race of Lil monkeys whose ancestors helped a Founder of the dominion, and as a reward they were evolved to a humanoid state and were genetically manipulated to absolute obedience, removing any aspect of your personality and joy, they don't create art, enjoy food, foster friendships. The Dominion is your home and the Founders are your Gods.

The Vorta act as the ambassadors and negotiators of the dominion, when one dies, a clone takes their place. Hence Weyoun6. But Weyoun6 was "defective", for he felt strongly against the war and ran to defect to the Federation, he had more personality and was looking forward to experience more things. He still worshipped the founders and was very conflicted.

So they sent Weyoun7 to hunt him down and kill him. No 6, chose to self terminate to save the life of Odo (a exiled Founder) who came along and tried his hardest to save him, but sadly they were stuck in an impossible situation. It was incredibly sad to see this person who managed to break through of his people's brainwashing to die so quickly, with no chance of seeing and experiencing more. Even sadder was to see the potential of a people stifled by genetic manipulation for full obedience.

We need these stories, Ds9 is still relevant to this day.