You're not a terrible human being, but you are not a wise one either. Once you let your employer know that you've been looking, they are going to start looking to find your replacement. You've told them that you are looking elsewhere so they are going to protect themselves against your eventual exit.
They may offer you the 10% more that you are looking for, but they will still be looking for someone else. It's only the rare exception that a company will offer a raise in response to an employee telling them they have an offer from another company and mean it sincerely.
Usually the response is "Uh oh, Sam is interviewing, let's plan for the worst". Then they start transferring responsibilities from you to other staff and eventually let you go once you've been made redundant.
No, it doesn't make you a bad person, but it's not the best way to go.
Instead, approach your employer and get let them know what the average salary is for your field and ask them for a raise. If you are at least an average employee, you have the right to ask for that much.
Don't play games though, that will only hurt you in the end.