Throughout his life, de Quental oscillated between pessimism and depression; afflicted with what might have been bipolar disorder, at the time of his last trip to Lisbon, he was in a state of steady depression, compounded by spinal disease. After a month in Lisbon, he returned once again to Ponta Delgada around June 1891. On September 11 that year, at about 8:00 PM, he committed suicide by two gunshots in the mouth while seated on a bench in a local garden park. He died approximately an hour later. "Of all things, the worst is having been born," he wrote in a poem.