What died was yuno's human form. In the novel, she returned back into a goddess after her "death". What broke the MC in ED1 was how the fake hero and his CHFs reacted. The fake and the girls coldly refused to help the dying yuno, and showed zero sympathy or remorse about her death. Later, after the grieving MC buried yuno, he saw the fake and the girls having fun at the bar, completely dismissing her death. Yuno's death, and the party's cold dismissal of it, was what finally broke the MC. If not for yuno contacting the MC via a dream that night to tell him to keep fighting, the MC likely would have unalived himself. The fake and the girls' part in the death of yuno is a primary factor into why the MC chose to punish them all in route 1. After the MC executed the fake, the girls attempted to apologize and explain to the MC what really happened, but instead the MC used his authority as the true hero to conscript the girls into what were effectively suicide missions, refusing to listen to whatever the girls had to say. Driven by extreme guilt and shame, the girls threw themselves into these suicide missions in a desperate bid to atone, wrecking themselves physically and mentally.
In ending 2, the forgiveness ending, or neutral route, the main difference was rather than punishment, MC wanted nothing to do with the fake or his CHFs. The MC had no interest in punishment, but he had no will to forgive either. When the girls attempt to apologize, he harshly rebuffs and brutally berates them, recounting everything they did, and their part in Yuno's death, leaving the girls completely distraught and sobbing. It is only later that the knight commander reveals the full truth to the MC. Learning the truth, the MC realizes the girls are victims and chooses to forgive them. In the end they restore their friendship, but the bond they had before was gone. They try to move on from what happened, but its clear they're all still haunted by what happened, and grieving what they lost.
Route 3, or the rescue ending, was where Yuno survived. Here, the MC was able to save Yuno from the incident that killed her in the previous timelines. After being saved, Yuno tells the MC the full truth, about his status as the true hero and what the fake did to the girls. The MC, having learned the truth, chose to rescue his friends. The MC outs the fake to the king. The MC overpowers and captures the fake, taking the hero's sword. With the sword in hand, the MC proves his status as the true hero. The fake is forced to release his control over his victims and is imprisoned. The MC and yuno later comfort the MC's traumatized CHFs. After the defeat of the demon lord, the MC marries Yuno and has twins with her.
In route 4, the true timeline(Vanilla Harem Ending), Yuno restarted the timeline, making so the fake was never summoned in the first place. The MC gets his hero powers from the start When the hero summoning ritual is performed, the summoned hero was the fake's sister, a mischievous but kind girl named Shouko. The story becomes a vanilla harem adventure. The MC and his CHFs eventually confesses theirs feeling for each other, MC pledging to marry all of them after the demon lord is defeated. He also consummated his relationship with his fiancee. During the final battle, the MC sees yuno, who was applying the final fixes to the world. The MC briefly remembers the events of the other timelines, and he and yuno declares their loves for each other, before separating. Years after the demon lord's defeat, the MC has married his CHFs, shouko, and a royal magician named Narufe. He has fathered a child from almost all of them. While having some time alone and reminiscing on his life, he still feels like something's missing. That's when yuno appears before him, ready to be with him at last.