As-Salamu `Alaikum dear sister,
Thank you for sending us your question. I want to begin by saying that I am very sorry that this happened to you. It is very apparent that you had no intention having a relationship with a man who has already been married. It is obvious that he was not open and honest with you from the beginning of your relationship and shame on him for his dishonesty and lack of integrity and respect for you, his wife, and for himself.
I cannot tell you why he did what he did. The reality is that there are many people in this world who behave with integrity and respect and others who do not. It seems from how you have described this man, he is from those who do not behave with integrity and respect. Please do not confuse this man with the values of Islam because he clearly has crossed the lines.
You mentioned, “Forgive my ignorance on the Islamic traditions, but I know in my Christian upbringing adultery is very frowned upon, and I am devastated that I was tricked into this situation.” Just as adultery is frowned upon in Christianity, it is also forbidden in Islam. It seems like this man did not care to follow Islamic teachings, and that is entirely his choice. He has (and will continue to) hurt many people by his poor choices, and you have every right to feel devastated and betrayed by his dishonesty.
From your written question, it is apparent that you are appalled by what he had done, and you are seeking to understand why he acted the way he did. Unfortunately, we do not always know why people behave the way they do. He is the only person who could answer that question. It does seem apparent that based on his actions he probably lacks insight into his behavior, and, as a result, has made a lot of poor choices. The choices that he makes in his life will no doubt affect him and the people around him, and you absolutely made the right choice by leaving him.
Please allow yourself to heal from this ordeal. It seems you are a person who behaves with values and honesty. Continue to live that way, and hopefully, you will be with someone who lives the same way that you do.
Peace be with you.
Disclaimer: The conceptualization and recommendations stated in this response are very general and purely based on the limited information that was provided in the question. In no event shall AboutIslam, it’s volunteers, writers, scholars, counselors, or employees be held liable for any direct, indirect, exemplary, punitive, consequential or other damages whatsoever that may arise through your decision or action in the use of the services which our website provides.