Obsessed with causality, a mathematician creates future simulation software only for it to predict he’ll die in three days unless he is prepared to kill a complete stranger, who is somehow connected to his fate, but instead he begins falling for her

“Using mysterious prediction software granted to him in his father?s will, a failing business owner discovers the day he?ll die and must kill a woman he?s never met in three days to alter his fate, only for things to become complicated when he begins to fall for her.”

Okay, good, you have a clear goal. To alter his fate. But how can we root for him if he’s killing an innocent woman just so he can live?
I think you should change the goal from killing to something else entirely. Killing someone to save one’s own life just isn’t a very sympathetic goal.

Since you include a romance angle in your logline, I shall do the same in my example.
Example:?When a prediction software he inherited from his father tells him the day he’ll die, a failing business owner must seduce a woman to have a child to alter his fate.?(~31 words)

So, good goal, but I suggest finding a different action.

Even in stories with villains as the protagonist, the reason the become villains is often?because of noble or sympathetic reasons. Or they go on the idea that the story is showing the villain’s side instead of the hero, meaning that it’s just a story about bad people doing bad things.
But the upcoming “Suicide Squad” makes my point. The usual villains, in exchange of early release, take up black ops missions that others will not do. They are still villains, but they are put into a familiar role: an action hero type role.
If your character started more sympathetic it would be fine. But the goal from the start is to kill a seemingly innocent woman. Because some software told him to. So if he kills her, what if he dies anyway? What did she die for? If he doesn’t kill her, would he even die at all? He is killing someone to save his own life, and he doesn’t even know truly how she?factors into his death.
If from the start the woman were a known bad guy, it would be better. He’s stopping a bad person, and he gets a benefit.

“A mother has three days to find and kill a total stranger identified by an uncannily accurate future simulation app as?triggering a chain of events that will inevitably kill her infant.”

I like this. The character is not killing just to save herself. She is killing to save her child. Something most people would do. Especially a mother.

Though, to cut down some words here’s my go:?When an uncannily accurate prediction app shows that her newborn will die in three days, a woman must kill the stranger who?triggers the chain of events which kills?her child.?(~31 words)

Or, more similar in structure to dpg’s:?A mother has three days to kill a stranger who is identified by an uncannily accurate prediction app to trigger the chain of events which kills her newborn.?(~28 words)

The title also has a nice ring to it. Actually, this version suggests that the character and the family has nothing to do with the creation of the software, that it’s just an app anyone can download. I think that would be an interesting idea to include in a story: How the world reacts when they can learn their future. Or, having more to do with destiny, if one is truly makes their own future.