Logline: A hardened cyber crimes analyst must shut down the source of a cursed smartphone app that seems to kill people who use it before the app strikes its next targets, her estranged family.

Well....it kills some users and not others. If you forward it to a friend you are spared. Would you say that qualifies as "an app that kills people who use it" for the purposes of the logline? It sounds better but is incorrect.
The app does not kill everyone who uses it.

SSalvatore, this sounds like "976-EVIL" but with smartphones, check out the movie as part of your research.

>>> "seems to kill"
Either it kills users or it doesn't. If it only seems to , but doesn't, there's no plot. So, of course, it kills. Period.