How did the detective stumble upon the case of the serial killer? What set him off on his “adventure” so to speak? I think the logline needs an inciting incident, an event that triggers the detective’s search for the killer, to clear that question up. Did he stumble upon one of the killer’s victims perhaps? Or maybe it’s something completely different. Either way, it’s something to consider.

“before the killer discovers the detective’s dark family secret” is great because it gives the story a ticking clock. The hero/protagonist has to acheive his goal (catch the serial killer) before it is too late. And that also makes the logline more compelling.

Because the wording for the character’s stakes (the secret that he/she doesn’t want to get out) is so vague in the logline, it becomes hard to imagine what consequences would strike the hero, would he fail. And we need to know what’s at stake if the main character fails to really root for the character to win.

Hope any of this helps.