You had broken a vow that you made with him. The two of you weren't supposed to say "I love you" until his goal was achieved, but you did it. You convinced yourself you cursed him by doing so. Is this a symptom of survivors guilt or did you truly curse him? Suguru was your companion for 7 years before his death at the hands of Yuta Okkotsu and Satoru Gojo. His ultimate goal was to rid the world of non-sorcerers, or what he would call them, "Monkeys". Now that he's dead, his goal falls to you.