Honestly, it doesn't seem too dark a quest at all, but in Fallout 4, when investigating the disappearance of Earl Sterling in Diamond City, and on encountering Sterling's mutilated body in the cellar, you kind of get this feeling that it wasn't all sunshine and happiness in the Jewel of the Commonwealth.
Crocker made a huge mistake because of his obsession, got guilt-ridden, mutilated the body, hid the body, and when you encounter him he wants to end it all because he can't handle the guilt of killing someone.
It got dark pretty fast, and even though it's just a simple and small sidequest, it got pretty heavy towards the end.