Midoriya acts like that because he wants to be a true hero. Heroes do not act vindictively or hold grudges. They need to strive for the betterment of everyone. If he was cruel or mean to Bakugo in turn that would help nothing. It would just make Bakugo angry and more isolated. If Midoriya acted that was he would be exactly what Bakugo fears he will he. A rival that will crush his dreams. By embracing him and supporting him, Midoriya has helped foster the change in Bakugo. He has taught him that even if you made mistake, even if you did something wrong, you can still be forgiven, can still be a hero. If you think empathy and compassion are weaknesses, I feel bad for you.