This is incorrect. movement is the delta from the previous mouse event. You need something like this:

var lastMouse;

function onMouseMove(event) {
    var xDelta, yDelta;
    if (navigator.webkitPointer && navigator.webkitPointer.isLocked) {
        xDelta = event.webkitMovementX;
        yDelta = event.webkitMovementY;
    } else {
        if (lastMouse == null) lastMouse = {x:event.clientX, y:event.clientY};
        xDelta = event.clientX - lastMouse.x;
        yDelta = event.clientY - lastMouse.y;
    }
}