I kind of agree with babyblue about how it seems more of a happy and "up" song for the most part. There are a few parts where the mood meets the darkness of the lyrics when things change and get a little slower like from 0:38-ish to 1:05-ish though.
First listening to the song, or just listening and not thinking of the lyrics at all, I get from the song a more happy and upbeat kind of song or story. But after a look at the lyrics, I look at the song a little closer.
I think they chose a more uppity beat to this song to keep that racing feeling or sense of a rush.
Here I'll show you. The lyric translations I got with the help of google translater. Not sure how accurate these are but its close enough, I think, to show you why they chose the music they chose for different parts of the song. Listening to the part of the song along with the lyrics and my analysis/interpretation of the song may help you better see what I'm talking about.
At the beggining we hear that happy and upbeat mood and music that is carried out with these lyrics.
(beginning to 0:35)
J’ai forcé la porte des enfers......................I forced the door of hell
J’ai rompu le sceau défendu.......................I broke the seal defended
Pour toi............................................... ..For you
Détourné le cours d’un volcan....................Turned the course of a volcano
Pour réchauffer tes bras tremblants............To warm up your arms trembling
Pour toi............................................... ..For you
Then things change a little bit when these lyrics are being sung. The music turns a little "darker".
(0:38 - 1:05)
Le ciel est noir.................The sky is black
Ma vie est sombre............My life is dark
Il gèle à pierre fendre........It is freezing cold
J’ai froid..........................I'm(It's?) Cold
Then again with the "happy", "upbeat", or "racing" mood of the song for these lyrics.
(1:06 - 1:29)
J’ai avancé l’heure de l’aurore.............I advanced the hour of dawn
J’ai brusqué le retour des fleurs...........I hurried back flowers
Pour toi..........................................For you
J’ai fait les vitres, changé l’ampoule.....I made the windows, changed the bulb
Etouffé les cris de la foule.................Smothered the cries of the crowd
Pour toi.........................................For you
Then back to the more down part.
(1:30 - 1:57)
Le ciel s’effondre.................The sky is falling
Débris, décombres...............Debris , rubble
Il gèle à coeur fendre..........It freezes at heart split
J’ai froid............................I'm Cold
Then things change more to what I would call a "hopeful mood" to the song for these lyrics.
(2:10 - 2:35)
Il n’y a plus un bruit dans le jardin d’hiver...............There is more noise in the winter garden
Nous nous abritons sous les ramures contraires.......We took shelter under the branches contrary
Then back to the darkness. And the ending.
(2:36 - end)
Sans toi, le monde..........Without you the world
Vacille, succombe...........Flickers, dies
Il gèle à coeur fendre......It freezes at heart split
J’ai froid.......................I'm cold
Sans toi.......................Without you
So as you may see, the blue lyrics take place in the more "happy" tones of the song and the dark red lyrics take place in the "darker" parts of the song.
In the first two blue parts we can see why they may have chose this "up" beat for the song. It gives that sense of rush or go, go, go. In those sections, that is supported with lyrics such as "I forced the door of hell", "Turned the course of a volcano", "I advanced the hour of dawn", "I hurried back flowers", and "I made the windows, changed the bulb". All these things would be done in a hurrying manner. But it all can be interpreted or taken differently.
The red parts we hear the music change to better match the lyrics such as "the sky is black" and "the sky is falling".
But this is just how I see the song and this is what I have found through my analysis of it. It can all be taken differently depending on who you are and how you understand or interpret it I guess.
I don't know French so when I listen to this song I still sometimes will think of it as more of a happy song rather than what the lyrics suggest
.