You'll remember the first ending song of Attack on Titan, where Mikasa is standing in the garden of knives. This scene signifies : Mikasa, the person who loved Eren the most, is the one who must kill him to stop his destructive path and free him from the suffering he brought upon the world. The knives symbolize the pain and sacrifice Mikasa faces as she carries out this tragic act. Their was a literal line in the first ed (go and check it out) -" now, sleep/rest in peace" from the song reflects Mikasa’s final farewell to Eren — it’s her way of saying that he can finally rest after everything he went through. In contrast, the seventh ending song shows Eren standing in the garden of flowers, representing the peace and freedom he finds only in death. The flowers symbolize the release from the torment and violence he experienced throughout his life. The difference between these two gardens highlights the irony of their roles: Mikasa, who loved him so deeply, is the one who must end his life, while Eren, who fought for freedom his entire life, only achieves it in death.