If you're too lazy to look at the cinematics on Youtube, here are all the spoilers:
-Everybody died in the old timeline except Shao Kahn. All the games you played are meaningless./
-Raiden sent a really cryptic and stupid message to hsi past self. It was probably cryptic and stupid because he was getting his skull bashed in at the time.
-MK1 pretty much goes the same as before. Nobody dies except Sub-Zero the Elder. Quan Chi does stuff.
-MK2 mostly goes the same, but doesn't. Raiden is convinced that his future message of "He must win" means somebody besides Liu Kang has to win.
-Ermac tears Jax's arms off.
-Kano sells Shang Tsung a rocket launcher. Is not chained to a wall.
-Raiden saves Smoke from the Lin Kuei cyborgs. Sub-Zero gets roboticized instead, and the Lin Kuei ally with Shao Kahn.
-Kitana turns good because Mileena is naked.
-Raiden does not allow Liu Kang to fight Kahn because "he must win." So Kung Lao kicks Kintaro's ass instead.
-Kung Lao gets murdered by kahn. Liu Kang goes Super Saiyan and punches through Kahn's chest.
-Kahn lives through this because Quan Chi gives him some good "homeopathic" healing.
-Mortal Kombat 3 happens because resurrecting Sindel lets the Outworld guys use a loophole to invade, not merge the realms.
-Stryker is actually useful for once. He proves this by beating up Reptile and Mileena, who never won any of their fights anyway.
-Kabal is a cop now.
-No wait, he's an ex-gangster.
-No wait, he's both.
-No wait, he's Darth Vader.
-Raiden kills Motaro by chucking him in the Hudson River. This is actually important.
-Cyborg Sub-Zero turns good, and kicks EVERY BAD GUY ASS AROUND.
-Kahn panics because Motaro dies, and ganks Shang Tsung to give Sindel lots and lots and lots and lots and lots more evil power.
-Nightwolf shoves Noob Saibot into Kahn and Quan Chi's soulnado, thus ending Kahn's attempt to steal souls.
-Liu Kang and Raiden go to the Elder Gods to ask "Why aren't you helping? NO FAIR!"
-The heroes beat up, and probably kill Cyrax and Sektor.
-Immediately after, SUPER SINDEL attacks the heroes.
-Sindel kills Kabal
-Sindel kills Stryker
-Sindel kills Jax
-Sindel kills Smoke
-Sindel kills Cyber Sub-Zero
-Snape kills Dumbledore
-Sindel kills Jade
-Sindel kills Kitana
-Nightwolf sacrifices his life to kill Sindel
-The good guys return. Liu Kang goes "wtf? ALL OUR HEROES ARE DEAD!"
-Johnny Cage and Sonya say "We're still alive!"
-Liu kang says "All the heroes are dead."
-Johnny and Sonya say "but we're okay!"
-Liu Kang says "All the USEFUL heroes are dead."
-Johnny and Sonya stfu for the rest of the game.
-Liu Kang chews out Raiden for screwing stuff up. Raiden runs off to do stupid things.
-Scorpion makes a cameo appearance. Raiden kicks his ass.
-Raiden asks Quan Chi to resurrect everybody.
-Raiden sells everybody's souls to Quan Chi.
-Turns out Quan Chi already has everybody's souls. Either way, he turns 'em all into zombies.
-Raiden kicks all their asses anyway.
-Raiden realizes that "he must win" meant "Shao Kahn must win. i.e., Shao Kahn must be tricked into merging the realms without a Mortal Kombat tournament. While simply conquering the realms was a loophole, actually merging them would violate the Elder Gods' "mortal kombat" rules, thus bringinmg them down on Kahn like a load of bricks.
-Liu Kang hears about Raiden's plan, decides that Raidne's off his gourd, and tries to kill him.
-Raiden "accidentally" kills Liu Kang "by accident." He performs this "accident" by repeatedly frying him with lightning, leaving him a horribly burned corpse.
-Johnny and Sonya do nothing.
-Raiden tricks Kahn into merging the realms. The elder gods go "lolno" and kill Kahn FOREVER AND EVER AND EVER.
-Johnny and Sonya look at Liu's corpse and are sad.
-Happy end!
-No wait, Shinnok's gonna invade.
So. DEATH TOTAL:
The Heroes: Everybody except Raiden, Johnny, and Sonya
The Villains: Kahn, Shang Tsung, Motaro, Sindel, Noob, (probably) the cyber-ninjas.
Honorable Mentions: Most of the villains lose all of their fights. Shang Tsung and Kano never win squat, but still remain threats. Same with Goro and Kintaro, who do an awesome tag team against Cyber Sub-Zero (Subby wins). But special mention goes to Reptile, Baraka, and Sheeva, who are complete jokes.