In the year that never was, the Master abused lucy physically. This can be seen because it becomes obvious that Lucy despised the Master in last of the time lords. She has a black eye and got revenge by shooting the Master.
As for why... Well, he's the Master, an insane psychopath, driven even crazier by the ever-louder sound of drums. He probably had fun playing the loving husband for a while, then got bored of that and had fun slapping her around for a while. He's not a nice guy.
No once again! The power of being God of Earth went o his head, so he lacked the genuine love he had for Lucy.