In just three lines, the speaker admits to wrecking their own life. Nothing outside has done this to them. The fire is something they set themselves, and the title, "I, Ruined," points the same way: the speaker is both the one ruined and the one who did the ruining.
The central image is a fire that keeps going until the fuel is gone. "I ran out of gasoline" is oddly matter-of-fact, almost like someone running dry on a road trip. That plain tone makes the destruction feel worse. The speaker doesn't stop because they've seen sense, but because they have nothing left to burn with. Then comes the last line, "I watched it all burn," where they go quiet and simply look at the damage.
The mood is bleak and exhausted. There's regret here, but also a numb, hollow calm, as if the speaker is past shouting and can only take in what they've done.