Why is Alice's Adventures in Wonderland considered dark?

Answer 1

Many critics have noted that Alice in Wonderland is set in the summer months, takes place mainly outdoors, and features mainly characters who are indifferent to Alice or sometimes helpful.

The later Alice through the Looking Glass however takes place in Winter, indoors, and has many characters who are actively hostile to Alice. (The underlying idea of Looking Glass - a game of Chess - contains an idea of combat).

When Lewis Carroll wrote Alice in Wonderland he knew Alice Liddell and her sisters well, and was on good terms with their parents. By the time he came to write Looking Glass he had become estranged from the Liddell family (though he later rekindled his friendship with Alice). Also by the time Carroll was writing Looking Glass Alice' sister Lorina (who appears briefly in Wonderland as the Lory) had died.

It is Looking Glass which is considered the dark book.