The (more) Modern Prometheus.
Chiefly I recall that creepy dualistic other watching over Charlie's actions, and goading by his mere presence.
This is an exploration of an open invitation to adulthood, bringing the challenges of greater complexity and ambition. The rewards of which are less than satisfying, and greater complications.
I don't think the ending invoked sadness. Ruefulness is more like it. I don't recall making the connection of Algernon's fate telescoping to Charlies' untimely death shortly past the story end. It felt more like both Charlies lived and died within the story itself, before the ending.
It's been a long time since I read this, sometime before high school. I may have missed things, and I'm sure I would have a much different impression if I read it now. I've carried this much remembrance of it with me, so I think I can say it's an excellent read.