Ephraim Kosciuszko owed nothing to the Empire.
He was tired of running to fight another day, and this was to be his final stand. For himself. For his people. For his planet.
From a young age, he had dreamed of being in politics. He grew up destitute and without family, and throughout his early life, worked tirelessly to achieve his endgame of being in the room where it happened. He fell in with revolutionary elements of society, giving impassioned speeches in bars and cellars listing the people's grievances with the government, and when the Glorious Revolution occurred, he made a name for himself as a soldier of great renown, fighting against a government he viewed as corrupt and unable to safeguard the rights of the people. He eventually was elected as the representative of his planet, the Outer Rim world of Pagalbos, in the Galactic Senate, once Pagalbos was accepted into the Republic, and during the Separatist Crisis building up to the war, he fought new battles in the Senate to reach settlements with the seceding systems, in order to keep the peace in the Republic.
During the first year of the Clone Wars, he was a leading figure in the movement for peace, whilst distancing himself from devout pacifists like the Duchess of Mandalore. However, once war came to Pagalbos, he resigned the office to take up arms once more, and did so under the lowest possible rank, distinguishing himself in combat against the Separatist droid army, and proving to be a tactical genius, achieving victories despite overwhelming odds. Once it was clear that Pagalbos was a lost cause for the Republic, he decided to continue the war, but by adapting the army into a guerrilla group, creating a myriad of a thousand problems from just one.
At the end of the war, the Grand Army of the Republic, along with a Jedi general, was fighting a conventional war on Pagalbos against the droid occupiers. When Clone Order 66 was issued, Kosciuszko watched as the Jedi general he had grown to respect was killed without mercy by the clone troopers he had served with time and time again, and despite having serious reservations about doing so, he came back to the Senate to serve Pagalbos and its interests.
Such interests were not shared by the Empire. Any dreams of freedom or autonomy were dashed with thunderous applause. Kosciuszko himself knew what had happened to the Jedi, that Palpatine was a Sith Lord, and that from the ashes of Anakin Skywalker, the Emperor's newest enforcer, Darth Vader, emerged. He also knew of the circumstances behind the death of one of his closest friends in the Senate, Padmé Amidala of Naboo, and mourned not only her death, but that of democracy.
For all of his rebelliousness and upstart activities within the Senate, Kosciuszko never thought that it would have to come to war and secession.
But indeed, it did.