Existentialism is essentially about having to make choices. Every action and every non-action is the result of a choice to do, or not to do, something. The summation of those decisions - to act or not to act - is you - who you are.
"I was only following orders". Not so, you have the choice to follow or not to follow the orders. Only YOU can make the choice.. Your life is a succession of decisions. And those decisions create who you are.