Because existentialists believe in radical personal freedom, everything is a matter of choice. We choose our values, we choose our identities, and we even choose to be alive. In creating and continually re-creating the self, man must choose a set of values through action. It's important to note that action, not thoughts, beliefs, or aspirations, constitutes this choice. By choosing a certain value (honesty, freedom, remorse, etc.), we create it. For Sartre, to avoid choice is to flee from personal freedom, and to engage in bad faith or self-deception.
Committing to murder Aegisthus and Clytemnestra is the first real choice Orestes makes in his life; it is the only decision manifested in action.