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.

I find that you are basically telling me in the logline that maybe an ex is killing off all the woman he is making amends with. To me a logline should never tell the outcome. I also would possibly keep out the brain tumor and allow the viewers to discover that info. I like richiev but maybe instead of mentioning the terminal illness you can say fighting time or fighting his own his own battles. I personally would just keep out his illness and make it mysterious in the logline.

Those movies were terrible.

It's not a stupid idea, see "Fight Club", however badly executed viewers may feel cheated at the end.
You need either to leave clues during the movies and show them during the big reveal a la "The Usual Suspects" or you need a bitter sweet end a la "Jacob's Ladder" so viewers will concentrate on that rather than the twist.