Rust has some hallucinations and loses time, but he understands that he's doing it, and there's no attempt to make the viewer question what is real or not.
Someone is stabbed around 38 minutes in, and another person is hit with an axe a minute or so later. We see someone pulling a knife out of a wound just after.
In chapter 3, Marty forces his way inside his ex-lover's house, physically assaults and threatens her date, screaming and pointing incapacitating spray at his face while she shouts at him to stop and leave.
Around 51 minutes, a husband puts a hand around his wife's throat as if to choke her during an argument. He doesn't hurt her, but it may be triggering.
Interpretively; the entire season focuses on a collective of politically privileged, caucasian men, orchestrating a three decade long sprawling cover up for dozens of abductions, rapes, torture, possible sacrifically motivated murder and serial killing of socially vulnerable children and women. A routine "chain of command" system, from the state governor down, is instituted for serveral generations; making it an untouchable family tree.
In the last couple episodes, dogs are threatened with physical abuse and murdered. In S1:E8 @ 0:04:45 - 0:05:05 a (cast iron?) object is thrown at a German Shepherd, possibly injuring it causing it to make a distressed yelp in fear. In the final episode S1:E8 @ 0:28:15 a dog is verbally abused on an off until 0:29:15. When it gets loose, it makes a terrible cry in pain (S1:E8 @ 0:29:25 off camera). At S1:E2 @0:29:48 the camera pans past the bloodied body of the animal for about 4 seconds; the body is shown at a distance/partially obscured again 0:32:57
The last episode of season one (form & void) is pretty messed up & graphic. There are several shots that imply animal abuse. Dogs and cats. Very graphic. If you have to watch it skip minutes 28-37
Several victims are found to have drugs in their system, and one abuse survivor describes the feeling of being drugged, but it's not shown or simulated onscreen (excluding characters who use drugs voluntarily)
there is a video tape presented in the show of a young child being raped. there is a cut scene before anything graphic is shown, but it is clear what has happened and could be triggering to some.
Reference in chapter 3 of someone being castrated with a razor in jail, description in chapter 4 of a similar (lengthier explanation) torture and murder method used by a drug cartel involving cutting.
Several characters are unlawfully beaten by police officers throughout season 1. Rust at one point describes a nasty cartel torture method, but it isn't shown.
Around 47 minutes in, Marty tells a story about a baby dying in a horrible way. We see the crime scene of the baby's death, and while the body itself is out of focus, there is blood and the scene is disturbing.
Rust visits a young woman who he rescued from abuse as a child. She is now in a psychiatric hospital and when he mentions the facial appearance of someone who he believes abused her, she has a panic attack and cannot stop screaming. She defintely has PTSD from her trauma
other comment is not true, the killer does not have d.i.d., he does put on different accents and is mentally ill, but is not implied to have d.i.d. at all
Mental illness is implied in two characters. The focus character has features of some altered personality disorders. A female character very obviously has a form of unspecified intellectual disability, potentially autistim and congenital disorders, which aids in her groomed exploitation and complete social isolation.
An inmate slits his wrists in his cell off-screen. you see the aftermath. A detective also tells a woman who murdered her children that she should kill herself in jail.
If harm to babies is a trigger: around 47 minutes in, Marty tells a story about a baby dying in a horrible way. We see the crime scene of the baby's death, and while the body itself is out of focus, there is blood and the scene is disturbing.
Several times in season 1, Rust either discusses or monologs about the meaning of life and existence, or what happens after death. There is a nihilistic bent to it.