fired for a personal matter

I am a manager who did business with a co-worker outside of work, he wronged me and I left a scathing message on his cell and he felt threatened and took it to HR and my boss. It was not a business related at all, I did not do anything at work. I had a exemplary file at work, I have a lot of kudos from my customer and the people I work with, my crew. I am going to take this individual to small claims, anyway an investigation was started, they suspended me and now the fired me because of the message I left on his voicemail.

Can they fire me for this? It was pretty bad language since I was very angry at what he did. I am indirectly this co-workers manager.