Thanks Chang! What you said about the scoring system is absolutely right. In the upcoming update plans of the game (not coming out yet), we decide to replace the score with stars:
1 star = finish the level;
2 stars = finish the level without being found out;
3 stars = finish the level and kill all the enemies without being found out.
Hope that would give a better evaluation for the players' performance. And yes, we will definitely add a more through tutorial!