I'd argue Tooru lost due to a combination of greed, humanity, and just not knowing something important.

Greed because he went to confront Josuke in an attempt to convince him not to destroy the medicine. This got him in a situation where he could be hurt.

Humanity because he held off during his attack because he wanted to give Yasuho a chance to leave without seeing him and getting on Wonder of U's hit list. This saved Josuke's life, and also gave Josuke the chance to attack him directly.

And then he didn't have the whole picture of how the Spin works and that Wonder of U would not automatically defend itself against the Spin's ability to shoot moving holes. Had he been either less greedy, or less human, he would have won.