Simply by pressing W alone, my character will already be facing the same direction as the camera; whether I'm pressing LMB or not has nothing to do with it.
Conversely, if I press A, D or S, my character will turn to face the corresponding direction (relative to the camera), then pressing LMB while moving will not change that direction (relative to the camera).

If this not what you're expecting to see then I didn't understand what you're trying to achieve.