How can someone be so cruel and have no empathy?

My partner of 3 years cheated on me, after telling me time and time again “I am not like those other guys”, “I would never do that to you”, and “I’m too old for that shit, I want to start a family” (the girl he cheated with is younger than me!!) - he knew that both my previous relationships had ended the same way, with me being cheated on and knew how much it affected my mental health and self confidence, trust issues etc. I am genuinely stumped as to how he has sat there, hid things and lied to me for the past couple of months? I could never put someone through that.