From wiki:
With its 0.22-second attack rate and the max channeling time, the Death Ward can attack up to a maximum 37 times. Can deal up to 318.18/477.27/636.36 (Talent 522.73/681.82/840.91) damage per second to a single hero.
I’m tired of getting people playing Witch Doctor who don’t understand that Maledict + Death Ward is not necessary to kill a support. Death Ward is likely to kill a full HP enemy support in 4-6s, making Maledict not only unnecessary but also increasing WD’s mana costs and making it easier to cancel than by using a simple Q->R combo. Or alternatively, if you are playing WD and have an ally core going 2v1 on a enemy support, it is likely that Q+E with the damage output of your ally is enough to kill a enemy support without needing to use Death Ward.
Learn to preserve mana and to not put skills on cooldown when they are not needed.
It is not only about mana efficiency but cooldown as well. A WD that used all his spells to secure a kill that had 90% of happening without Maledict or Death Ward means there is at least a 18s timing that I have a walking creep around me who carries a 300-400g bounty upon killing. And since early on the death timers are low enough, this means that securing a kill on a enemy support is barely worthy.