In a recent blog post on the Eidos Montreal community page, staff member Adam Badke stated the following:
Players will have multiple ways to approach their objectives and each objective can be reached using Garretts 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!
the blog in question.