Why do some guys cheat , flirt with other woman, and still lie even when they get caught cheating?

I have a boyfriend of 2 years now, I met him online through a dating site. We lived in the same country and his home is 1hour travel by train from my home. He is 9 years ahead of me. Im 43 widower. Our first few months in relationship everything goes well. I didn't doubt that he will cheat me cuz i trusted him. He is divorced and living alone. Got son but living to his mom. I always go to his house every Sunday during my off. He always asked me to buy foods and asked me to clean his house. Iron his clothes while he is sleeping. I tidy up his house and even cooled for him for his dinner. He always hides his phone behind bedroom door. And i decided to checked it and found out a chat from other woman named Mercy saying she is tired hungry and need banana. My tears flowed silently and walked away in the room and put back his phone else if he found out that i touched he will slap me and bit me. When he wakes up, i asked if he knows a woman named Mercy. He told me no. I keep quiet and just smile. I let it go. Then yesterday, Sunday i found out again a chat from Mercy saying that she is tired, pain and banana. Does it mean that they already have sex? I told him that if he didn't stop chating her, I will leave. But he told me, I buy the laptop powebank first before he will stop chating Mercy. ( Note:: he always asked me to buy him things like powebank , clark shoes but i won't ). He always called me fucking idiot, slut , stingy woman,. Then he blocked me when i confronted him. And won't chat me at all. One time he threaten me that if i touched his phone again, he will chase me out of his house. He always bite my thighs and tell me he want to put a mark so that when i cheat , the man i met will know i have boyfriend. I dont chat other man at all or even meet. Cuz i knew the feeling of being cheated. My question is, why he need to lie, cheat me and flirts with someone else even i showered him true care, love and even understand him most of the time?

Why do some guys cheat , flirt with other woman, and still lie even when they get caught cheating?
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