Déchire comes from the verb déchirer, which literally means "to tear" or "rip" something, like a piece of paper.
In this song, it's used to describe a powerful internal state: "Ta raison se déchire" translates to "Your reason is tearing itself apart". This violent imagery perfectly captures the theme of intense mental conflict and the feeling of losing control, which is central to the song's title, "Zombie".
GIMS kicks this song off with a classic case of overthinking. He describes a battle inside his own head: "Ma raison somnolait / Ma conscience me conseillait / Mon subconscient m'déconseillait" (My reason was dozing / My conscience was advising me / My subconscious was advising me against it). While all these internal voices are arguing, his spirit just wants to break free and fly away. It's a feeling of being pulled in a million different directions by your own thoughts.
He feels completely disconnected and numb, like he's just going through the motions. He says he's trying to build a life but it feels like he's "construis dans l'vide" (building in the void). When people offer him simple advice like "L'espoir fait vivre" (Hope keeps you alive), he just asks, "Comment faire vivre? Je suis un zombie" (How can I live? I am a zombie). He feels like he's lost his true self, that he's become "l'ombre de toi-même" (the shadow of yourself), manipulated and paranoid. The song is a powerful look at feeling emotionally dead inside while the world keeps turning around you.