Curate, connect, and discover
If you want to include dark subjects and/or difficult topics in your writing (character death, addiction, mental health, SA, abuse, etc.) then you have to dive into the consequences of the topic - never shove it into the story just for the sake of drama, stakes, audience retention, or whatever other reason that doesn't have to do with the story itself.
To use one of the "lighter" examples here, let's so you decide to kill of a character - for the sake of simplicity let's call this character A. Now that they're dead, how does A's absence affect the story? How do other characters feel and cope with A's death? Who's grieves and morons A? Who's denying that A is even gone? Who's numbed and confused about A's death? Who's happy about A being gone? How long does it take for everyone to settle into a new normal without A? What does this new normal look like for them? What makes A's death actually important instead of a background character that no one would notice is gone?
These are all very important questions to ask if you're going to kill of a character, with each piece making the characters feel more alive and making A's death have actual weight behind it. To ignore these questions, leads to a flat world with characters who feel inhuman for how they can move on form the ultimate end and A's death feeling unimportant and something you'd only remember when looking over a wiki. This also means having characters die at the end of the story a half-assed choice. Don't get me wrong, it can still work if handled well - one way to handle this well is focusing completely on character reaction instead of the long running consequences. It's just that many times I've seen characters dying at the end of the story is just to ramp up the stakes, so their death isn't lingered on for long enough for it to feel like it has any importance other than being recognizable characters being the ones dying.
This advice runs true for all examples of dark subjects and difficult topics. You can't just have traumatizing things happen to characters and then not bother to look into the permeant ramifications of that trauma, along with just saying a character has a mental illness, not bothering to research the mental illness, and instead show only a stereotype of a mentally ill person. Not properly showing the consequences makes the world feel flat, the audience feel bored or annoyed, and worst-case scenario cause legitimate harm to those engaging with your work.
This is a rather common mistakes for first time writers to make, they tend not to fully think about the consequences of certain scenes and how it will permanently affect the story. If you've made this mistake before then that's absolutely okay, just remember to ask these impotent questions in the future.
Okay, fair warning, this last paragraph as less to do with actual advice and more me complaining about this mistake happening on a professional level even though if you've been in this creative field long enough to make it into a career you should know better by then. Okay? Okay.
But, and this is what led to me thinking over all this stuff and wanting to give out this advice to the world, this is also a common mistake made by middle aged or older men who are professionals in their creative field to make towards female characters and the topic of SA. I think this mistake stems from these producers/writers viewing SA as a "female trauma" (incorrect but you can't expect much of these dudes), so if they have a female character that they need to give trauma they'll just go with SA and not care about the long terms ramifications. Meanwhile male characters get family death, commonly his wife and/or child[ren], as their main trauma and it's still not treated with much respect - it's mostly just treated as the motive for why the male character is doing what he's doing and the dead wife and/or child[ren] are never shown having a life or personality of their own that would really make their death feel like a tragedy outside of the relationship with the male character, which is a disservice to the character(s). If you're going to make a character, or multiple characters, someone's whole motive then you should at least try and get us to care about the character on their own.