First, the answer: the quote is from Mark Twain, taken from The Mysterious Stranger. The character who speaks the line is a blond teenaged ("so young and beautiful") boy angel named Satan. I asked about the year it was written, and it seems like a range of 1896 up to Twain's death in 1910 would have to be sufficient.
I did a little digging on The Mysterious Stranger. Most academics today consider it an unfinished novel, Twain's last big project. The version I read was published posthumously in 1916, but is viewed with contempt by many Twain scholars nowadays due to the heavy editing and invention of characters by Albert Paine, the biographer who had sole control over Twain's manuscripts during that time.
Okay...it looks like there were three separate versions that are floating around, which were what occupied Twain's last decade-and-a-half. All three, as well as the Paine mis-mash, have as themes Twain's feelings about the hypocrisy of organized religion, the Moral Sense, and the "damned human race." What I read is a very serious social commentary.
One version is about an angel named Satan, a young teenage boy and nephew/namesake of the famous fallen angel, and his travels throughout a 1590 small village in Austria. Satan tries to enlighten the kids about the Moral Sense, the ridiculous way mankind treats mankind, etc. It ends abruptly with Satan entertaining in India, likely because Twain stashed it in a drawer to work on something else, probably another version. This was titled in his notes The Chronicles of Young Satan.
Another version, titled Schoolhouse Hill, starred Huck Finn and Tom Sawyer and they're adventures with Satan. This is the shortest version.
The third version returns to medieval Austria, but Satan's name is now No. 44, and has No. 44 out to explain the futility of humanity to the watching kids. This version has Twain playing with his late-in-life ideas about a "waking self" and a "dreaming self," a duality of selves that interested him. In his notes this is called No. 44, The Mysterious Stranger: Being an Ancient Tale Found in a Jug and Freely Translated from the Jug.
The version I read, the one put together by Paine and published as a novella in 1916 as The Mysterious Stranger; a Romance, has as the bulk of the story The Young Chronicles of Satan (though heavily edited with the Astrologer character wholly added by Paine), with the ending of the No. 44... story hap-hazzardly slapped on in a weird precursor to ontological science fiction of later times.
I have to say I was heartened to see that the ending was from a different manuscript entirely, since I didn't think Twain would have so abruptly switched gears, going from a tale about an unfeeling and powerful angel to a philosophically dense ontological dilemma about one's self in the course of a half-page.
Get an idea of how Twain saw his fellow humans...check out the story.