Actually, as the Supreme Court ruled in 1869, the southern acts of secession were not legal. Therefore their actions were a rebellion and the Lincoln administration acted properly in putting it down.
But let's assume for a moment that their actions were legal. Since the Davis government formally declared war on the United States on April 17, 1861 then the actions of the Lincoln administration would have been acts of self defense against the aggressor, wouldn't they? How can you complain about that?