Firstly credit to poster RewardBucks for his previous post of free comics on the Amazon US site than can be read through Kindle.
https://www.ozbargain.com.au/node/302863
His post was more geared towards superheroes and when going through all the comics available noticed quite a few more that might be of interest to readers especially of an apocalyptic/sci-fi/horror bent.
I'll just make a general list and these are for the most part full issues not previews and all issues are #1 in their series unless specified.
These are all currently free and some are larger than expected.
Transformers: All Hail Megatron #1
Warhammer 40,000: Will of Iron #0
The X-Files: Season 10
Star Trek: Countdown
Star Trek/Planet of the Apes
Rise of the Planet of the Apes Prequel: Chapters 1-6
John Woo's Seven Brothers
Street Fighter V (FCBD 2016)
13 Coins #6
Brutal
Chosen
The Darkest Hour
The Chronicles of Dr. Herbert West
Edge of Doom
Guy Ritchie's Gamekeeper
H20
Doc Macabre
Banshee: Extended Edition
Ramayan 3392 AD
Ramayan 3392 AD: Reloaded (Free Guidebook)
Kill Shakespeare #1 & #2
Dead Soldier
Warlash: Dark Noir
Amanada Hocking's The Hollows
Armor of Light: Tactics Anthem
Rumble
Lurkers
Wildfire
G.I. Joe: A Real American Hero #155-1/2
Remains
Nicolas Cage & Weston Cage's Voodoo Child
Wake The Dead
30 Days of Night
Secret Skull
Hitman: Agent 47: Official Movie Prelude
Witchblade #80 & #131
Wormwood: Gentleman Corpse
Dead Island
Rob Zombie's Bigfoot
Snakewoman
These are mostly free promo comics - ie they're free all the time.
BTW the Transformers All Hail Megatron's page count is issues 1-6. This is labelled #1 not Vol 1 so I suspect it's either the 20 page first issue or a mislabeled Vol 1 in which case it's a bargain.