Not a matter of "excuses", but I think an insanity plea could be entered under the circumstances, especially considering the tragic background combined with a genetic trigger, that causes mental health deterioration nearly 100% of the times.
I am no Sasuke's fan (my favorite character has always been Naruto), but fair is fair. Sasuke is without a doubt the most complex and interesting character of the story. While he did bad things, he only killed either villains or in self defense (samurai), so far. He has been such a remarkable failure in killing good guys (Naruto, Sakura, Karin, Killer Bee, Kakashi, etc.) that one has to wonder if he really did want to kill them. If anything, the Karin episode signified it. He stabbed through her after ensuring it would not be fatal (or why would he tell her not to move?), then as soon as Sakura showed up, tried to push the unpleasant task on her.
A more interesting question is what was different about Shisui and Itachi that allowed them to overcome, or suppress the darkness (certainly there was a lot of darkness in Itachi, but it did not overpower him), and my guess is that it was again the power of love - having something you wish to protect at all costs. Both Obito and Madara became truly evil only after they lost that special person in situations perceived as betrayal by their comrades - Rin by Kakashi, and Izuna's sacrifice disdained by the Uchiha - respectively. I think Sasuke hasn't gone all bad because he still have that special bond with Itachi and Naruto, and has not lost it through betrayal. Both Itachi and Naruto declared they will be there for him no matter what - and willing to back it up with words. I think that's what is keeping him sane.