This is really great that you uploaded a video! This way I could see that in your animation something is wrong with the root motion. Normally, when you view an animation sequence in the animation editor and have root motion enabled, the root would stick to the center [0,0,0] and the character would “moonwalk” on the same place. But because your character snaps back, this either means that your pelvis bone snaps back in the middle of the animation, or that your root motion is failing. When you uncheck “enable root motion”, does your character walk forward and forward and forward? Also, did you retarget the animation from the original zombie asset to the dummy skeleton? Perhaps something went wrong with that. You can also show the bones to see if the root is moving (there should be a red line when you uncheck “enable root motion” like in the picture from https://docs.unrealengine.com/latest/INT/Engine/Animation/RootMotion/
http://docs.unrealengine.com/latest/images/Engine/Animation/RootMotion/RootMotionBone.jpg