Aside from the fact that Meursault tells us that he drank tons of wine during their beach picnic, there are other hints that Meursault is not in his usual state of mind as he has been before.
In this chapter, for the first time we see Meursualt taking sides in an argument instead of just passively watching and calling things "interesting." Perhaps being drunk makes Meursault forget the fact that he's supposed to be "emotionless." Not reacting to things emotionally must take a great deal of thought and effort, and in his sober state Meursault is used to being able to filter out his emotions completely. But, once he is drunk he is no longer able to filter things normally.
This is evidenced through the fights with the men who follow Raymond and Meursault to the beach. Meursault seems to have some sort of emotional investment in the fights because instead of standing back and not fighting he takes a side with Raymond. This is a first, because in other conflicts he has never really intervened but merely watched (e.g. when Raymond beats his girlfriend and when the old man beats his dog). In the beach fights, though, Meursault looks out for Raymond by yelling "Look out, he's got a knife!". This not only shows that he was on Raymond's side, but that he was somehow emotionally invested enough to want to warn Raymond of the weapon.
Also, the way Meursault describes the beach and the hot summer day is very surreal and almost dream-like. This is because he was drunk and not seeing the world as clearly as before. He comments on the fact that he has a piercing headache and can't stand the hot sunlight. Maybe this was because he was dehydrated and the alcohol in his system also caused him to get sick. (I looked this up, and apparently drinking too much on a hot summer day makes you more likely to get heat illnesses.)
Perhaps Meursault's drunkenness is the reason why he chooses to shoot the man four more times than necessary. This is also an example of Meursault feeling emotion because he says that the four shots created unhappy emotions for him: "Then I fired four more times at the motionless body where the bullets lodged without leaving a trace. And it was like knocking four quick times on the door of unhappiness. "
All the same, I don't think that the fact that he was drunk makes his crime any less serious. It would be false to say "Oh, well. He was drunk so he didn't actually mean to kill anybody." What I do think is interesting, though, is seeing this second side to Meursault where he shows at least a small bit of emotion (fear/judgement/unhappiness).