Aries: To save a family member from death, typical martyr sacrifice.
Taurus: Suicide. It's as simple as that, whether it's they couldn't handle a burden, or they killed themself before anyone else could.
Gemini: Drugs, after a traumatic story of some sort for a past, they drowned themself in the only way they could feel free, drugs.
Cancer: Ya ate too much- you were in a food coma that was so intense that you never came out of it and doctors had to cut your life support.
Leo: A disease, any disease that you caught, developed or was born with kills you in your anime.
Virgo: Alcohol, seems cliché but after a lifetime of fighting, they let go, even if it's only once and they don't stop letting go until they're dead, free of the burden called life.
Libra: You aged, you were probably everyone's favourite old sensei but you grew too old and died but at least you weren't short lived and it was peaceful
Scorpio: In a violent fight, whether said Scorpio was on the good or evil side, their opponent and them were evenly matched, somehow the opponent found a way to gain the upper hand, of course both fighters would be in critical condition at this point, it would only be fitting that the Scorpio didn't need to suffer any longer.
Sagittarius: Poisoned, you could kill and defend at any other time but you were in a vulnerable position when eating or drinking, so much that your enemy took advantage of that.
Capricorn: In a heroic way, only it was for a lover, would have never sacrificed themselves in their entire life but knowing that the one person who cared and loved them was about to die, they would take their place instantly, leaving them dead.
Aquarius: You were willing to have a civilised discussion with a villain, but that made you naïve and gullible because they pushed you off of a cliff.
Pisces: You died in a hilarious accident of some sort, the people that watched your anime death cried but at least they had a smile on their face.