I have the urge to use my company's open source code to compete with them.
That seems shady, why would you use your company's code?
It was a successful project, so I think it would be a nice marketing stunt.
If the work is open-source then it's fine to use it. Otherwise you shouldn't steal from a business.
So I'm stealing if I use the previously created code base. Aren't I evil if I use it?
I guess you could consider yourself evil if you do use it. I would highly recommend you don't do this, it would most likely end up with you in legal trouble.