An oddball couple turns a simple break up conversation into a fatal encounter when ulterior motives for the split start surfacing.

Could you specify who of the couple will be the main point of view character? In other words is there a main character in the story?

If not then it will be very similar to War Of The Roses and as such will likely need an external character's point of view to tell the story through like the Dany DeVito character.

The Ulterior motives description is too vague to be useful in a logline, why not specify exactly what each of them wants so it is clear what their actions will be throughout the story?

Hope this helps.

I think it's too vague. I would be more interested if it were a little more specific. How does a break up ever become fatal? I agree that it hints at comedy and then hints toward horror / thriller.

When you say 'oddball couple' it hints to the reader your story will be a comedy
But the rest of the logline seems to indicate the story isn't a comedy.
This makes the logline a bit disjointed.