A serial womaniser discovers women from his past are murdered soon after making amends and must find their killer before an inoperable brain tumour kills him.

In view of the success those movies (Thirteenth Floor, So I Married An Axe Murderer, Showgirls) have enjoyed, I think that you are in the minority finding them terrible.
Please give us a list of non SF movies about mental psychosis or with an unexpected final twist at the end that you consider good.

"that maybe" / "should never tell the outcome"

You are still unsure, nothing has been 100% revealed.

Besides, this is just the logline, I haven't even decided who the killer is yet.

"fighting time"

Fighting time how? This is vague and uninteresting, the reader needs to know 'how'.

"keep out his illness and make it mysterious"

I've already taken out the illness.

Also, the great part about having similar story lines to terrible movies is that you can learn what made them terrible and turn your story into something great! Good luck.