Last year I started working as the graphic and web designer of a call center. After several months, my boss opened a marketing agency where I was the first guy working there for 7 months. I'm a graphic and print designer with strong knowledge in web design and the new agency didn't need anyone else at the time when it was a startup.
After some months, they hired 2 other employees: one developer and another graphic designer. I managed the team to the best of my ability, and was promised the manager position at the end of the next month.
The developer was not really experienced and the only reason they hired that guy was because he was the only one who wanted to work there for that price. On ALL of our projects I had to help him just to get my own work done.
One day my employer informed me that the new manager will be the developer even though they promised me the position. The reason given for the decision was because of his work performance, a reason I highly doubt.
The situation quickly deteriorated after that and I told my employer I will quit at the end of the month. I found a new job 2 weeks after, but 2 days ago I got a call from one of my bosses that they fired the developer/manager and are now offering the management position to me with double the pay. I'm hesitant to accept this offer to work there again. I'm happy with my new job but they offered me an attractive salary.
How can I evaluate this offer after initially being declined?