Bishop Vitaliy Skomarovskiy stated that every believer has a duty to defend their own life, the lives of their loved ones, and the common good of the people. Therefore, the believers of the Roman Catholic Church have been defending our state and fulfilling their military duty in the ranks of the Armed Forces of Ukraine since the beginning of the war (since 2014).
This is discussed in the article by Glavkom.
The fulfillment of military duty, not only in a state of war (mobilization), does not contradict the religious beliefs and doctrinal teachings of the Catholic Church. Every person has the right to defend their right to life. Therefore, by defending their life, we do not become guilty of murder, even if we are forced to kill an attacker (Catechism of the Catholic Church, para. 2264). Therefore, killing an enemy while defending one's country is not a sin. The Conference of Roman Catholic Bishops of Ukraine openly refers to the Russian Federation as an aggressor.
By the way, the Ukrainian Orthodox Church also does not consider killing occupiers as a mortal sin. "The destruction of Russian attackers by defenders of Ukraine who commit armed aggression against our people and try to seize the land given to us by God cannot be equated with the sin of criminal murder. This is a forced, just and sacrificial defense based on love for one's family, people and Homeland," the comment of the Ukrainian Orthodox Church states.