I don't see Willy as a psycho. To the contrary, he's the sanest person on board. That's what makes him so dangerous. He knows what he wants, knows where he's going. Everyone else is confused, agitated, prone to petty squabbling, only pull together, act as a team, when they kill Willy! The film was controversial at the time, viewed by many, probably rightly, as an allegory of how fascists rise to power, take control, while the liberal democracies fidget about, don't really know what to do with all their freedom. The fascists, they have the answer: they seize power.