I don't know if this flashback is necessary or not. The whole point thus far is that Towa was content in sacrificing himself for his friends. And that was something essential for him as a character becasue of his prior hatred towards humans. His hatred birthed from his mistreatment from his family that was eventually overriden and reborn as love towards his new "found family." It's about him coming to grips with his friends and the world moving on after he died, and him convincing himself that he needs to move on and live his life with Fia-mama.
I can see only one compelling reason to include it. If those memories hold something that gives him the courage to reach back out to Arios, Liliana, the Succubus, and the Elf, then the flashback earns its place. Otherwise it risks feeling like backstory for backstory's sake. Because I dont think this is a story about him just living out his days with Fia-Mama so the flashback has to serve some narrative purpose other than just exposition.