Why in some cases, things goes out of control between the customer and employee when the customer is angry ? And how to handle angry customer ?

First thing to remember is that the customer is not dealing with the company but you. For customer you are the company. Please take pride of this! 

Second thing to remember is the rule No.1 - The customer is always right. Rule No.2. When the customer is wrong, please go back to rule No.1

Emotion should be under control and only empathy should be shown. None other than one of our colleagues made customer become agitated. We have to resolve this instead of defend ourselves.

Listen! Listen!! and Listen!!! you will start understanding the customer's feeling and start respecting. 

- If the customer is particularly angry, he or she may say something (or several things) that are really rude to the employee , which drive the employee to defend himself and give agressive replies.

in my opinion this is the fuel for things to go out of control.

- Most important, as employee you should keep in mind that you should not take it personally-- he or she is upset with the system, business, the product, or the service they have been provided with-- they are not upset with you as a person. You will have to set your personal feelings aside, remain calm and adjust your mindset. this should help not to fire the situation up.

- Greet them and welcome them with smile 

- Listen actively to what the customer is saying.

- Repeat the customer’s concerns.

- Actively sympathize.

- Take personal ownership, and apologize.

- Be engaged and offer a possible solution (or solutions), and take action immediately.

- Check for satisfaction and go beyond their expectation .

Talk to the customer politely.