Because if you do not stop the violence, it will continue. Ukraine is defending itself from an invasion. Providing the tools to resist the invasion saves lives. If Ukraine were to stop resisting, Russia would murder millions of people like they have already tried to do.
Following that, Putin and his cronies have declared that they would invade Poland, Lithuania, Latvia, Estonia, Czech Republic, and every former member state of the Soviet union. Many of these nations are currently NATO member states which would directly kick off of a Third World war and which lead to nuclear war in Asia and Europe.
Russian can peacefully and the war that anytime by withdrawing. Why haven’t they? They are the instigators and they can end it.