If Mulldrifter was cast using evoke, it doesn't become sacrificed immediately upon entering the battlefield (just as two cards aren't drawn immediately when Mulldrifter enters), but rather when its evoke triggered ability resolves (C.R. 603.2, 603.3, 117.4, 608.2c, 702.74a). You can cast Undying Evil in response to that triggered ability, and Undying Evil will get to resolve (and give the targeted creature undying) before that ability (C.R. 117.7, 117.1a). See also this thread.
EDIT (Aug. 22): Some rules were renumbered in the meantime.
EDIT (Aug. 22): Some rules were renumbered in the meantime.