Ask HN: Unethical to finish project from a company that never finished it

I worked on a mobile application for a project that I was really invested in but after graduating, different developers with different coding beliefs - the project just fell apart. I still love the idea and put so much time into it. Has a lot of potential I think. Can I just complete it on my own. Do I even have an obligation to ask them if I can do it? When is their idea no longer "thiers". Also did not like the direction they were going with the business.