I see Stardust acting like this because he feels tremendous guilt for cutting his friend's finger and blames himself for being too "weak," which is why he adopted this mentality that he has to fight with his full strength. If he doesn't, then he is afraid that there will be even more casualties.
When Spirits comments that Stardust does not feel any emotions, I think she is wrong in assuming that he lacks empathy. Rather, his guilt has caused him to close off his heart because he is afraid that if he does let anyone else in his heart, they might end up hurt.