Which Sci-Fi work first showed artificial destruction of a planet?

1915 (if moons count): "John Jones's Dollar", a short story by Harry Stephen Keeler; first published in the August, 1915 issue of The Black Cat, a scan of which is available at the Internet Archive. The text of the story is available at Project Gutenberg.

"To continue. In the year 2714 A.D., an important political battle was concluded in the Solar System Senate and House of Representatives. I am referring to the great controversy as to whether the earth's moon was a sufficient menace to interplanetary navigation to warrant its removal. The outcome of the wrangle was that the question was decided in the affirmative. Consequently—

"But I beg your pardon, young men. I occasionally lose sight of the fact that you are not so well informed upon historical matters as myself. Here I am, talking to you about the moon, totally forgetful that many of you are puzzled as to my meaning. I advise all of you who have not yet attended the Solaris Museum on Jupiter to take a trip there some Sunday afternoon. The Interplanetary Suburban Line runs trains every half hour on that day. You will find there a complete working model of the old satellite of the Earth, which, before it was destroyed, furnished this planet light at night through the crude medium of reflection.

"On account of this decision as to the inadvisability of allowing the moon to remain where it was, engineers commenced its removal in the year 2714. Piece by piece, it was chipped away and brought to the Earth in Interplanetary freight cars. These pieces were then propelled by Zoodolite explosive, in the direction of the Milky Way, with a velocity of 11,217 meters per second. This velocity, of course, gave each departing fragment exactly the amount of kinetic energy it required to enable it to overcome the backward pull of the Earth from here to infinity. I dare say those moon-hunks are going yet.

An earlier entry in the "if moons count" category would be The Struggle for Empire (1900), by Robert William Cole.

When the attacking Sirians push the (Earth-based) Anglo-Saxon Empire defenders back to Jupiter, an unexpected side-effect of the weapons used in the long-running battle (so definitely artificial, but not intentional) causes 2 of Jupiter's moons to collide, disintegrate, and disperse into a cloud of material, part of which falls onto Jupiter, with the remainder eventually condensing into a single mass.

[...] The giant forces which had been let loose on that occasion were not exhausted in the battle area. They interacted upon one another until a vast vacuum, a space absolutely devoid of ether, was formed, and this slowly travelled through space as a great wave. It sped on and on without anyone being aware of its existence or the destruction it might occasion. It passed through the orbit of Neptune and rolled on until it came to Jupiter. Then it passed over the two moons between which the Anglo-Saxon and Sirian fleets were contending in deadly combat. The result of the vacuum was that the two moons were driven towards one another by a colossal force. Most of the officers and crews were suddenly startled by observing that the two discs on either side of them, which had hitherto been so small, were rapidly getting larger. Nobody could make out what was happening; some thought it was merely an optical illusion. But the discs rapidly grew in size from minute to minute, until they covered almost half of the heavens.

[...]

Larger and larger grew the moons and brighter the light. Now they occupied the whole of the heavens; there was nothing to be seen but mountain and forest rapidly expanding and spreading out before their eyes. The crews howled with terror, but the masses rushed on, irresistible, relentless. The outlying ships were caught up on the rocks and pressed on until they dashed against the others. Then, in a second of time, before anyone could move or speak, the two moons met with a fearful crash. Every ship was ground up, pressed flat, and destroyed. Mountains and hills were broken off and ground to powder; forests of trees were snapped off and torn to matchwood. The heat generated by the impact was intense. Lakes and rivers were immediately evaporated; but the grinding, crushing and splitting still continued. Mountains melted and were converted into incandescent vapour; whole countries were torn off and went bounding against one another, crashing and smashing, until they, too, were vaporized. There was a chaos of rocks, mountains, stones, and dust shaking, clashing, and rebounding. A cloud of vapour hung around and grew until it reached Jupiter, throwing a fierce heat and light all over the planet, and even lighting up the distant earth and Neptune. And so in a moment the two great fleets had been wiped out; not a ship escaped to tell the tale.

The damage done to Jupiter was immense. For hours a perfect tempest of half-melted rocks, jagged hills, and lumps of liquid metal fell on to it, committing frightful havoc. The great cloud of luminous vapour slowly rotated round it, withering the foliage of the trees, drying up the rivers, and scorching the face of the land. [...] But at last the excessive heat was dissipated, and the cloud gradually contracted, only giving out a mild warmth. The times and tides of Jupiter, however, were altogether set wrong by the catastrophe, and the orbit of the planet itself was altered by the displacement of the two moons.

_{The Struggle for Empire, Chapter XI: The Catastrophe at Jupiter}