Maude was selfish for...
Like any suicide, there's a selfishness in how the action may affect others who care.
Yes, perhaps the suicidal person is putting a stop to their own suffering, which may be more than the collective sadness others may feel afterwards, but here Maude wasn't suffering.
She knew Harold had emotional problems. She knew he was lacking proper connections to others in his life, and that he had invested all his emotional wellbeing in her. Their connection was two-way, but she didn't think twice about deciding to end it all on her own.
Selfish. I think if he really felt that attached to her, the more realistic ending would have been him killing himself for real. Not cynical, just realistic.