During the main story mission when you meet Michael in the sewers, there will be a disemboweled dog in the final room of appartment 109. They'll be lying in the upper right section of your view as you enter the room. There's really not much else in there so you can safely exit to the hallway from the doorway to your left.
Several of the special zombies definitely fall into this category. Even some of the regular ones can get quite gruesome, with open injuries and infected wounds even before whatever weapon you used to kill them further damages their bodies.
You can find weapons in the game which are quite literally called "Developer Weapons" and are meant to be joke weapons created by the developers. There is no in-universe explanation for these, although you have to go out of your way in order to find many of them.
In a side quest early on, you find a man named Dawud who wants you to get a gun. After you give it to him, it's said that he threatened his wife and forced her to give him their child before he ran off.
[SPOILERS] There is a side mission where a man’s wife is giving birth and you keep bringing him alcohol to disinfect. Once your character has had enough, you can lockpick their door and find them drinking all your booze you gave them!
[SPOILERS] During the penultimate mission, you start to hallucinate due to the zombie venom and you start joking your friend thinking they’re a zombie themself.
Corpses of infected people who hanged themselves can occasionally be encountered; usually in their homes, but they can be found in other locations as well.
Zombie and human enemies alike can die in very gory ways if you use powerful weapons, especially if they have mods attached. Blood also pools around their corpses.
The assumption of the other user that Gazi is implied to be autistic is rather unfortunate. There are clear hints he is in fact intellectually disabled, as evidenced by him asking for the film 'Charly', based on the novel 'Flowers for Algernon', about a similarly intellectually disabled man who undergoes surgery to increase his intelligence (only to realise he was far happier before the procedure) , and 'his mama saying the film was about him'.
(signed, someone autistic that doesn't appreciate intellectual disability being confidently confused for autism)
SPOILERS!
It depends. In the main story ending, you are met with some sad confrontations but it ends in you winning a confrontation with the main antagonist. In the DLC, The Following, however, the ending is sad and you end up turning into a zombie (or blowing up all of Harran with a nuclear weapon in the secret non-canonical ending.)
You can find remnants of car crashes in the street, and in the Following DLC you get a buggy which you can crash very easily in the beginning if you are not used to driving.