Did they cut the killer slack because he was drunk? That commonly happens, even though I disagree with it.
In cases like this, the killer is directly responsible for his own mental state, and should be fully responsible for whatever he does in that state.
Someone I knew received a higher sentence because he was NOT drunk when he whipped a guy's ass (the guy wholeheartedly deserved it), but had he been drunk would have been sentenced more leniently.
Ultimately, in this case, the responsibility for what is being used as an excuse remains with the criminal, and should not be regarded as a mitigating factor. Had he been organically insane, that would be different (as in something he could not control nor make a decision not to be.