In the opening credits, the characters are shown with Glowing Eyes of Doom. Other than making them look evil, the colors also works a a form of Foreshadowing of their fate:
Boar, Monkey, Horse, Snake, Dragon and Tiger are shown with red eyes. These characters are killed by the red-eyes Rabbit and his zombies.
Actually, the latter was killed by Ox, but because of Rabbit.
Also, four of them does end up as red-eyed zombies.
Ox is shown with blue eyes. He is killed by Rat.
Dog, Chicken and Sheep are shown with green eyes. They are killed by someone else (Chicken, Ox and Tiger, respectively).
Rabbit's eyes aren't shown. He commits suicide.
Rat's eyes are normal. He wins the tournament.
It's odd that the Tatsumi Brothers, twins, represent different Eastern Zodiac signs: Snake and Dragon. However, it's important to known that there is more to the Chinese astrology than the "Animal of the Year": the twelve signs are also assigned to the month, day and hoursnote called inner, true and secret animals, respectively of the person's birth. This means that someone born during the Year of the Dragon can have Snake as an Inner Animal, for example.
An simpler interpretation is that they born minutes from each other, but the Lunar calendar year has passed the threshold from the year of the dragon, to the year of the snake. The year of the snake comes after the year of the dragon. (i.e. Year of the Dragon: February 17, 1988 to February 5, 1989. Year of the snake: February 6, 1989 to January 26, 1990)
In the eastern culture, dragons are considered evolved snakes.
Also a case of Death by Irony, most of the character have deaths that echoes or mirror their killing style:
Boar kills in abundance. Her zombie corpse is "killed" in abundance, being devoured by an entire horde of crows.
Boar is also very confident on her skills and takes pride on being the last champion's daughter, so much that she killed her sister just to enter the tournament and prove her worth. What is the most karmic and humiliating way for her to go, if not being one of the first victims.
Dog kills by biting his enemies. He has his skull crushed by an "ally", Chicken, after biting her.
Chicken kills by pecking, and her ability is also called the "Eye of the Cormorant". She is stabbed in the eyes by Ox and devoured by her own crows.
Sheep kills with trickery/kills by deceiving others. He was fooled by Tiger's appearance and is killed by her, before he could react.
Horse kills silently. He dies quietly and alone withing the bank's vault, after Snake's corpse burns down the place, due to the lack of oxygen.
Snake and Dragon robs their victims as they kills. Rabbit kills them, getting possession of their bodies, weapons and abilities. Tiger also steals their weapons and use it against them.
Dragon has planned on using his brother as an distraction to launch an sneak attack on Ox and Tiger; instead, it was Rabbit who used Snake's head to distract Dragon, and reached him with Monkey's help.
Rabbit kills strangely/kills psychotically. He is killed-off for good while looking like an human abomination.
Tiger kills powerfully while drunk/kills in a drunken rage. She is mercy-killed by Ox, while weakened, sober and at peace.
Ox kills directly. His body is atomized by Sheep's grenade.
Rabbit's character design makes perfect sense, given his powers and fighting-style: The giant bunny tail can hide a pair of large knives, his sharp teeth exists so he can, successfully, bite his own tongue to reanimate himself as a zombie, and his weird choice of clothes intentionally makes him look silly, thus distracting others and causing them to underestimate him. Plus, if he became a zombie, clothes would just get in the way of his reanimated limbs and Combat Tentacles.
He targeting the Tatsumi brothers before the tournament also makes sense, as their weapons are one of the few things that can destroy his zombies.
Rat, surprisingly, got along with Ax-Crazy Rabbit in one of the Deleted Paths. In the Vietnamese Astrology, Rabbit is replaced by Cat (Rat's best friend in the Chinese tale) as the 4th zodiac sign.
Some events of the story are similar to the ones from the original Chinese Tale: Snake "scarring" Horse, Monkey making an alliance (and Chicken being one of her allies), and Rat using Ox to win the race.
Michio Tsukui/Dog has an adoptive daughter. However... now that he's been killed by Ryouka Niwa/Chicken, who's going to be looking after his daughter?
While Nagayuki/Dragon and Takeyasu/Snake are both horrible people, their situation is is pretty terrifying when you think about it. The only people that they care about at all in any way is each other and they have fought together as a team for their entire life. They were extremely excited to be able to fight in the Zodiac War and were sure that they would be able to win together...only to find out at the last minute that their family conveniently left out the fact that "killing everyone" includes each other and that they will be expected to fight to the death if they both make it to the end of the tournament. Even though they had time to process that fact and were pretty much resigned to it by the time the contest started, it still had to have been pretty horrible, particularly for Nagayuki/Dragon; how would you like to spend your whole life protecting someone and taking pride in doing so, only to be told that you need to kill that person?
Rat's power side-effect. During the entire tournament, not only he is stressed due to his mind being loaded with information for "participating" another 99 times, but he also recalls the pain from dying in the other routes: stabbed in the back by Rabbit, decapitated by Ox, gunned down by Boar, eaten alive by Chicken's crows etc. And when he finally wins, he gets killed at least 40 more times by Duodecouple, for avoiding the interview. No wonder he wanted to forget about it.
How Sheep expect to ally with anyone if he is a former champion known as "the Warrior that kills by deceiving others"? Also, by planing to show his Beast Gem to other participant to convince them that he can remove them, Sheep expect them to react with "Wow! This guy is a wizard! He can create wormholes inside your body!", instead of "You never ate your Gem, cheater!." or "You must have killed another participant and taken it."
For some reason, Chicken never though of ordering her birds to drop bombs to the tournament, despite doing so in her flashback.
When Monkey found out that Sheep had planted explosives beforehand, why did she move them to below floor instead of just disarming or throwing them outside? This leads to Snake's early death, with indirectly leads to Boar, Horse, Dragon, and her own death.
This one is directly addressed in the anime: She wanted to use the bombs for her own devices, so removing them wasn't in her best interest. She's the one who set them off in the beginning to give her team a chance to escape after sensing someone else's killing intent in response to her trying to make an alliance, which she expected would happen given she's trying to broker peace in an assassin based killing game.