Battlestar Galactica killed off several characters throughout its four-season run, but perhaps the most shocking death was Lt. Anastasia "Dee" Dualla's suicide in season 4. Billions of people died at the very start of Battlestar Galactica when the Cylons bombed the Twelve Colonies, leaving the remaining survivors to suffer in a nuclear wasteland. Thankfully, the Battlestar Galactica banded together with dozens of other ships, including one that carried the new president of the colonies, in order to preserve the human race and find a new home.
They spent years and fought countless battles with the Cylons - and sometimes themselves - searching for that new home, Earth. According to their scriptures, Earth was a mythical planet that the original Thirteenth colony lived on after they fled the planet Kobol. Instead of following the Twelve Colonies to the new worlds, they went their own way - and that's because they were a colony of Cylons. Sadly, despite a long, arduous, and deadly journey reaching Earth, the Fleet's survivors had to leave the planet since it was also a wasteland.
Immediately following the Cylons' attack, Commander Adama was convinced to abandon his plans to engage the androids in a war, and instead leave the colonies behind to find somewhere new to live. Given what people were going through, Adama bluffed and said he knew the way to Earth. That plan eventually became a reality - thanks to various events - but it remained the only hope people had. When the survivors realized they couldn't live on Earth and had to continue searching for a new home, they lost hope; some people, like Dee, killed themselves as a result.
Dee's death was especially shocking because she wasn't only a major character who had been around since the beginning, but she seemed to be rekindling her relationship with Lee Adama as well. In fact, she committed suicide only minutes after their date ended that night; and as she told Lt. Gaeta just moments before she died, she just wanted to hold onto that tiny bit of happiness for a little while longer, forgetting about all the problems that faced them - an ongoing war with the Cylons, finding yet another home, and so forth.
While that was the story reason, series creator Ronald D. Moore once told the Chicago Tribune that the behind the scenes reason for Dee's death was so there were consequences to finding Earth. He said, "There had to be a cost. There had to be a price somebody paid for that discovery. Not everybody could take that. Not everyone could just say, 'OK, that didn’t work out; let’s go on to next week’s episode.'" In a time before peak TV where characters in major shows are killed off every season - if not multiple times a season - Dee's death was certainly unexpected and difficult to accept for longtime fans.