Walker's job is to prevent people using time-travel to alter the past, thereby changing the future; this is what he's trying to prevent McComb, the film's main antagonist, from achieving. However, when Walker himself travels through time, he does not hesitate to cause as much damage to his surroundings as humanly possible. When he travels back to 1928, for example, he beats up two security guards, fires a futuristic laser-cannon in front of a large crowd of people, then jumps out of a window with the criminal in tow, and disappears into a wormhole above a busy street in New York City. Doing these things would drastically alter history, but none of these events seem to have any noticeable effect on the future/present.