Umm I don't think Paru necessarily glorifies those selfish actions. It's a bit like Riz. The story tries to bring up that sometimes, someone indeed can do fucked up things, and the reasons are not as simple. It's not as simple as "X is evil just because it's on his gene" kinda thing. Some people might be desperate enough to do things that "normal" people deem as illogical. When you are in a state of depression, simple logical things become a difficult task. And this theme has been around from the very beginning.
When Legosi meet Leano, he basically was still sad. He accepted why his mother do that, however he looked for some better way to avoid the same thing befall his future relationship. This led him to believe that the answer is by embracing the possibility of his kid be a hybrid, raising the kid with full love (to avoid the kid turn into Melon), and look for ways so hybrids can be accepted in society.
That being said, I also can understand how some people perceive the story as an act to glorify and accept such negative actions.