There is no need to explain if we are discussing Leo's route (implying that we have already read it).
Yes, Leo did show signs of toxic relationship attributes, but his "psycho[tic] and manipulat[ive]" tendencies are not without reason. He dearly and clearly loves Chase (probably too much), but he allow his fears and expectations to cloud his thinking and judgment sometimes hurting people in the process; he loses touch with reality when he imagines and fantasizes about Chase, for better or worse. But, can he really be blamed? Maybe his upbringing contributed to his insecurities? Are we any wiser in punishing a child for eating a forbidden fruit that we came to covet so much? No need to answer, just food for thought.
"Those that hurt the most need love the most." If I was Chase, I would have given Leo the chances to change and make better himself. They had a history, a childhood, and a relationship, as well as dreams and aspirations. They could have reached ad astra together. If not life-threatening, why cease all that over feeling uncomfortable? A part of life is about learning from mistakes and improving the self - molding into the ideal person for yourself and for others. Leo is deserving of love, he just need guidance, patience, and open-mindedness.