The difference, by the numbers
Both have Yellow Chartreuse. Only the Death Flip has tequila, Jägermeister, simple syrup and whole egg; only the Widow’s Kiss has Calvados, Bénédictine and Angostura bitters. In parts, a Death Flip is 2 tequila, 1 Jägermeister and 1 Chartreuse; a Widow’s Kiss is 2 Calvados, 1 Chartreuse and 1 Bénédictine. By volume, the Death Flip is 49% spirit and 49% liqueur; the Widow’s Kiss is 50% spirit and 50% liqueur. That makes them close cousins on the ratio map.
The Widow’s Kiss is the stronger drink in the glass (23.3% ABV against 13.4%), but being smaller it holds about the same alcohol (1.34 and 1.42 standard drinks). The Death Flip has more calories (about 231 against 175), but the Widow’s Kiss has more sugar (9.0 g against 7.4 g). Both are shaken and served in a coupe.