I think you both were being immature. Fenrir, you moved too quickly before thinking about -everyone's- feelings in the situation, and Leon was hasty as well.
Right now it's all high drama because it just happened, but I say give it time to cool down, then if you want Leon back in your life as a friend, then approach Leon again explaining that the feelings were all new to you, and you had no intention of hurting him. Of course right NOW he won't listen because you just probably broke his heart a little, and emotions were going everywhere.
Everyone needs to cool off. No one was "more in the right" than the other. Just looks like it is a big mess to me- people jumping from one partner to another is always going to be a bit messy.