That's an extremely tricky question, OG.
I regard the "person" as the product of the higher cognitive functions, while instinctive behaviours are controlled in parts of the brain we share with less advanced vertebrates.
We cannot yet identify any brain activity associated with the instinct-level "desire" to produce/obtain offspring, let alone the higher function of concepts like "wanting to be a father", due to limits of current technology. I cited an animal example involving destruction of the displaced pride-male's last set of cubs. I do not know to what extent that is genetically-programmed, instinctive behaviour or whether it is forethought and planning (higher cognitive functions) but at the animal level.
We humans may still have the same 'wiring', deep down, but have still higher functions (shared by other primates) where we learn and store behaviours - social 'norms' - with which to suppress those baser instincts.
Loss of control through alcohol is, obviously enough, impairment of those inhibitory nerve impulses. So, fair enough, I concede that loss of self-control can happen.
BUT:- New girlfriend has newly arrived baby. Surely, in a sober moment, he should have realised that the newborn means a new lifestyle, new responsibilities - no more getting smashed on 'n' pints of booze. Doing it in spite of that is criminally negligent.
If negligence can lead to a child's death without even touching it (hypothetical case), how would that be charged, just for comparison?