Why did he do this? Are we supposed to sympathize with him? Was this done out of depression?

Depression is part of it. But he hates the society he was born in. He wants revenge for forcing him to kill his creations. They killed his beloved creatures saying these creatures don't deserve to live - so forcing everyone else to prove they deserve to live is his justice.

Why does he love the creations, and why is he the only ancient who does?

Because everyone else is obsessed with "collective" thinking. Serve the planet. The little sparks on the planet mean less than nothing. Everything only exists towards a larger goal. Society teaches them death should be celebrated, sadness for death should be repressed - they had a cutscene showing this. He's probably not the only one but one of the most prominent, he's the one relevant to the story.

And why is he the only ancient who wanted to know what the meaning of life was?

Lots of Ancients are curious, but a lot of them are satisfied with the answer "to serve the planet". It is the dominant view in their society. Hermes doesn't accept that.