Spider vs. Butterfly

I found a Monarch Butterfly caught in a spider's web. My first instinct was to go rescue it. But then... I thought about it and wondered if that was the right thing to do. The spider caught this as food and he needs his lunch.

I looked closely at the spider. He already had something he was eating. The poor Monarch was struggling and fighting. I couldn't stand to watch so I went and released it from the web. I left that fly for the spider. It was already dead.

But I keep wondering. Was that the right thing to do? I know I shouldn't interfere with nature and the way it all works together. Somehow, I find that very hard when it is spider vs. butterfly.