Ok so I'm not really experienced with manga. However I am very hooked onto Sensei no Yasashii Koroshikata for how dark and brutal it is, and I'm feeling content starvation because updates are rolling too slow, and my digestion is far too quick... So I'm grateful for any suggestions. Here is the criteria, but it's more a guideline to what I like than something strict:

(1) I do really enjoy the female student/male teacher relationship in the story (makes my inner fangirl heart happy). I mean, age gap does it for me in general.
(2) I also enjoy bittersweet/cold themes, or even a slow burn to feel like the relationship is more realistic and developed to make up for that age gap lol. And by cold I mean it could be that relations are rocky between the ML and FL.
(3) The male lead is a gentleman...Prince-like even
(4) Brutality! Blood...murder...ahahaha....

Hope this information helps.

________________
Death is the Engine of Earth

It doesn't check all your criteria but try
Angels of Death bonus it's completed.

Thank you!!

________________
Death is the Engine of Earth