Being a lawyer, I am sure he knows of potential loopholes that he can try to wriggle through to be found guilty only of the lesser crime.
If he truly suffered a bout of it, why would he fill the tub with ice? Why wouldn't he just call the police and explain his actions? Wouldn't he just say that he suffered from it from the get-go rather than wait and see what his options were?
What he did afterwards, while being a crime in and of itself, does not alter the fact that the murder was or wasn't premeditated.