IF Kira is depressed, it probably has something to do with Gin's betrayal. However, there is actually little evidence that Kira is depressed. He has an introverted, quiet personality but he is cheerful with his friends, seems to enjoy many extra-curriculum pursuits - such as being a member of the Shinigami Men's Association, teaching poetry, writing poetry etc, etc. Certain Kira is serious presence in the battlefield but so are a number of other people. Just because Kira isn't all sunshine and lollipop, doesn't mean that he's home alone cutting himself. He just has a more subdued personality then, say, Rangiku.