If I had to take a guess why is the difference this vast when law should be the same for all, an abusive husband being killed more likely comes off as a ''planned act'' (like a knife taken from a kitchen drawer and used in an other room, ''I did not see an other way out and when he came closer...'', whereas abused wife being killed is a ''crime of passion'' (''Honestly, I don't know what possessed me, I was drunk, you know, I blacked out and when I came to my senses, she was lying in a pool of blood and there was blood of my hands, I will regret it for the rest of my life...'')
Edit, to make myself clear, I do not say this system is any right, and I do not believe the abuser actually wasn't in control, because they are always perfectly in control before police.