So far, I know I can kill him to advance. But I'd like to save the guy. So far, I only made it up to the flooded area.

I did find a broken(?) ladder next to the girl's cell, but I have no idea how or where to use it.
There's also a trap door near victim #1's cell, but I can't open it.
What I did found, though, was the waxed cylinder I can use on the broken grammophon close to the area where I need to save the second victim.

But what am I missing?

Here's a hint: How did you reach the area to find the wax cylinder?

Also, "saving" the victims is the same as "not killing them" in this scenario.

I spent some time too trying to figure out how to open his cell, but that's impossible. However there's a way past the room without killing him, but you'll have to figure it out.

After several hours of trying and looking around, I wouldn't ask for help, would I? :/

Really, you guys are not helping.

Look up.

Guys, you're forgetting the most important hint. Explore ALL of the passages, even the ones that lead to dead ends.

I thought he had already figured it out because he wasn't asking for more hints. I usually try to give more subtle hints than outright solutions and personally if I end up being hopelessly stuck and I have to search for walkthrough I prefer sites like UHS hints, because it helps you to the right direction without spoiling the puzzle.

No, I didn't, because my private life is currently very busy and I had next to no time to play anything. At all.
Also, I figured asking for more hints wouldn't help me and I'd get the same responses over and over, which would end in fighting. And I don't feel like fighting.

And usually, I take my time to tell that I figured out the problem on my own. I consider it rude to left a solved problem open.

I ordinarily would have suggested UHS, except for two problems: firstly, it did not occur to me at the time, and secondly, if OP's level of frustration is anything akin to what I have felt when getting fed up with a puzzle, doing anything short of giving the answer would not only not be helpful to them, but would also be severely irritating to say the least.

Whoa, wait? You can save the victims?

Game sure didn't go out of its way to make that clear.

Listen to the record at the start again: "Just remember: they can all be saved. There is always a way."

This game has SOUND?!

Salvation by irritating physics puzzle! At least there's some logic to it, the second victim on the other hand, hoo boy, good luck with that one. It involves sliding picture plates in the correct order. I never figured it out on my own, when I saw the solution, and tried to double back and figure out the logic behind it I couldn't find it. It seemed completely arbitrary. When someone provided an explanation on Steam it made sort-of sense but you'd have to really stretch to come to its conclusion. Third victim, well I was too busy RUNNING LIKE HELL JESUS CHRIST to stop for him so... sorry guy!

Anyway this definitely felt like a throwaway expansion. Unnecessary red herrings, two of its three major puzzles being really annoying (I don't care if they're optional, they're technically SUPPOSED to be solved) and the story completely pointless. The one thing though is that the "suitors" are in a way scarier than the monsters in the main game. Not just because, for the last two anyway, you can't just "wait it out" and the methods of escaping them are far more tense than any point I can remember in The Dark Descent (could partly have to do with the fact you start the whole game over if you die), but the actual nature behind them. Won't say anymore other than Justine is one horribly cruel beeeeitch.