Murder is a sin, killing is not.
Do you consider what you did to be murder?
If so go to confession and confess it to a priest and receive absolution.
There are good reasons for killing sometimes, self defense or the defense of others been two of them, such killing contrary to been a sin are actually good works.
If your not sure go speak to a priest and let him restore peace to your soul, he is the expert and will know what you need, a Catholic military Chaplin would probably be best if you still have access to one.
Whatever you have done in the past there is no reason for you to just quit on God and accept hell as your future, God’s mercy is greater than our sins and His forgiveness is always waiting for our acceptance.
Your eternal destination is not yet decided you have form now until you die left to influence it, the choice of final destination is still very much in your hands.
You might find it helpful to read about soldiers who later went on to become saints, St Ignatius of Loyola for example was a soldier like you who went on to become Saint, there are many more, St. George is another example then there is St. Maurice who is patron saint of armies and infantrymen.