You are right: The discussion "was Jesus a liar, madman or fool" on the basis of the gospels does not take into account that they were written after his death by a community of believers (although not centuries after his death). And those gospels differ in their presentation, especially John vs. the synoptics. If anything, he was a holy madman because he believed fervently in the immediate coming of the kingdom of God.
But it is neither true that the divinity of Jesus was invented and imposed by Constantine. There was a strong current in that direction from the beginning. The problem was the idea of redemption connected with the death of Jesus. This put Jesus in a central religious role. And in a monotheistic religion there is only one center, namely God himself. So Jesus became identified with God in some way very early, at least emotionally, as seen in the monarchian movement in the Roman community (sorry, couldn't resist that last bit).