It can be challenging to deal with a defensive insecure individual. They often appear confident and self assured until they open their mouth. They are usually self centered because they have not matured beyond the stage in life where they think everyone is thinking about and judging them.
The first step in dealing with an adult that is insecure and defensive is to recognize them as such a person. Look for the fast defensive responses that they may give to even the smallest criticism. I have encountered adults who are so defensive that they hear criticism even where it is not intended. To deal with these people you will have to have a lot of patience and tolerance. You will have to decide if they are worth putting up with. Do you have to work with them? If so you will have to use a lot of tact. You may even have to bite your tongue to keep from responding inappropriately to their pettiness.
The next step you will need to take, is to do everything in your power to assure this person you are not a threat to them. They may not be easy to convince. Remember this person has never grown up emotionally. You may just have to be straightforward if all else fails and tell them to get a grip.
Next, you may have to be a little nosy and dig into this person's background to find out just why they are so defensive and insecure. I have noticed that some people are threatened by others only if they see them as a threat on the job or in their friendships. If you are an outgoing friendly person who has a lot of charisma they may be especially threatened by you and respond in a childish manner. Be careful if this person is in a position of power and has the ability to affect your employment. Use your best judgement and try to get them to understand you are "not out to get them".
One last step to take is to try to be understanding if you suspect the person is not quite as intelligent as they have led others to believe. They will really feel threatened by someone they perceive as smarter than them. I am not suggesting that anyone should hide their intelligence. Just don't flaunt it where it is not wise. Use tact!