What does that have to do with Lincoln getting his brains blown out?
The suppression of the insurrection was legal. The War of the Great Rebellion was conducted with courts in session, and the Supreme Court made several rulings against the Lincoln administration.
By contrast the Davis regime of the pretended confederacy never got around to appointing their counterpart to the US Supreme Court. Davis was thus free to act illegally, and did. After the war was over, a southern partisan illegally murdered the elected US president, an act the victorious US did not do to the traitor, Jeff Davis.
Of course there is that rude song about the sour apple tree for the southern partisans to complain about.