They didn't kill it, they did exactly what they promised to do: they converted it into an Outlook sub-brand tool that speaks Exchange first and everything else second. It's just unfortunate that it was harder to keep Wunderlist's "soul" than Accompli (Mail) or Sunrise (Calendar) in the conversion/brand and feature alignment, but also maybe not surprising because TODO features have always maybe been historically some of the weakest parts of Outlook.