She lamped him (literally) while he was asleep, which could've done some serious damage, or worse, which resulted in him calling the police and saying out loud "She's trying to kill me", and on responding they find him injured but also now her covered in blood after self harming.
The whole reason they altered the law years ago was because there would quite often be sympathy for an abuser once things had calmed down, they'd sobered up, or they'd begged for forgiveness AGAIN.
But in a few relationships this would almost be standard and regular behaviour, which sometimes like in this case can go one step too far where the injuries involved call for outside intervention to put a stop to it.
Booze certainly does not help, but add drugs to the mix and that's where the violence takes a different turn as they will bring out the worst in you.
Most peoples claims about poor mental health state are 'normally' triggered by booze, drugs or more likely a mixture of both in the first place, so it's all self inflicted.
It would have actually been very wrong if the CPS hadn't pursued what went on with the evidence that was presented, but i kind of get the feeling that this would have ended badly anyway whether they had or not, then people would have been shouting at why they didn't get involved.