Alternative form of what doesn't kill you makes you stronger

Hardship builds moral character.

One who or that which kills. / (figuratively) That which causes stress or is extremely difficult, especially that which may cause failure at a task.

(slang) Excellent, very good, cool. / Causing death, destruction, or obliteration. / Distressing, uncomfortable.

(colloquial) A man unusually attractive to women. / (colloquial) An uncaring womanizer.

Alternative spelling of lady-killer

A sea mammal related to dolphins and porpoises (Orcinus orca).

(philately) A cancellation mark that is so heavy that it defaces the postage stamp to which it is applied, devaluing it from a collector's point of view.

A technological product that is so good, or so innovative, that it is irresistible to potential customers; especially a software product that persuades people to buy the hardware needed to use it.

(informal, computing) A killer application.