I'm still thinking about it. But this is where I stand right now: the older brother's response is understandable but not necessarily excusable--it's not something we should endorse. After all, he is bitter and angry. He doesn't believe the celebration is fair; his father's passionate love has deeply offended him. So he refuses to participate in this feast of love and forgiveness, clinging to his own morality, refusing to participate in the kingdom of grace. At the end of the day, he's not too sure he wants to be a part of a family where rebellious sons are loved and embraced without restitution, without paying for their mistakes.
In light of the context--Jesus speaking to the Pharisees--this is a legitimate interpretation, I feel, of the older brother. Remember, many of the Pharisees were scandalized by Jesus because of his message of grace. They were constantly on the outside looking in. They did not see their need to repent of their self-righteousness to become a part of God's new creation.
The older brother is like that. The father is pleading with him to come in--to enter into the kingdom of love, forgiveness, and transforming grace. But he refuses.
Such an interpretation pushes us toward the heart of the gospel. The gospel is not fundamentally about morality, nor is it about immorality. Rather the gospel is first and foremost about living abundantly in Christ. It's about being a part of God's new creation, about becoming a new creature in Christ--all of which comes from God's gracious activity in our lives. The gospel is not I obey, therefore, God must accept me. Such an approach is a prison because the question becomes, "How much obedience do I owe?" The gospel instead is, I'm accepted, therefore, I obey. The life I live, I now live in Christ. This is the only stance that truly enables us to love, forgive, and bless our enemies because it recognizes that all is from grace. Perhaps that's what the older brother forgot, he forgot that it's all a gift; it all comes from grace.
I'm thinking of Paul. If anyone lived a moral life according to the law, it was Paul: "as to righteousness under the law, blameless" (Phil. 3). But he continues: "Yet whatever gains I had, these I have come to regard as loss because of the surpassing value of knowing Christ Jesus my Lord. For his sake I have suffered the loss of all things, and I regard them as rubbish, in order that I may gain Christ and be found in him, not having a righteousness of my own that comes from the law, but one that comes through faith in Christ, the righteousness from God based on faith" (Phil. 3).
What matters, even for a former Pharisee like Paul, is new life in Christ and his kingdom.
Peace.