Don't blame yourself if the employee you accepted years back turned out to be a difficult one to control. You don't have a crystal ball to foresee what he will become after several years.
Don't waste time regretting your actions for there are many effective ways to turn him back to his fresh and innocent-minded self. Here are some suggestions.
1. Investigate
Even if it seems that your employee is difficult, don't just jump into conclusion. Investigate the problem as to when it started, how it affects the organization and to what extent.
The information you should gather must be objective so as not to color your judgment against the person.
2. Talk to the problematic employee privately
You must not project the image that you are only considering the words of other people against him, but show him that you want to hear his side on the matter. The purpose of this talk is to help him see his part of the problem and to make him commit to changing his ways if proven guilty. But give him all the chances to explain himself. In that way, he won't be able to say that you are biased against him.
Part of the reason he got that way is the fact that you did not intervene early enough. Perhaps you are too busy, or have thought that his problem will go away after some time. Now that he his problem has gone more serious, you can't waste any more time.
Talk to him and in no uncertain terms lay down all his options on the table: what he needs to do if he still wants to stay, or what awaits him if he maintains his difficult attitude.
4. Give him time to make the necessary changes
After your talk, tell him that you are still willing to give him a chance if he is willing to change. But he needs to prove it by his actions. If he does change, well and good, but if all else fails, then you must do what you have to do.
