According to Eidos Montreal community manager Adam Badke, the new Thief will accommodate players who prefer a non-lethal approach to their play-through.
In a post titled “Going Rogue” on the Eidos Montreal Community blogs, Badke wrote:
Players will have multiple ways to approach their objectives and each objective can be reached using Garrett’s various tools, by exploring and by playing with your environment.
Each path comes with a set of challenges, encounters and looting opportunities. Difficulty options allow the experience to be scaled for hardcore fans of the original series, but to also let newcomers have a lot of fun. And yes, you can complete the game without killing anybody – Garrett is a Master Thief, not a killing machine!
Considering the heavy emphasis on stealth in Thief games, this is far from surprising. But it does make the new Thief the latest in a growing number of games to not only include non-lethal solutions, but reward players for avoiding bloodshed (recent examples include Dishonored and Deus Ex: Human Revolution).