It’s always interesting to discuss the motivations or moral inclinations one could read in the characters of Death Note.
In most cases, as one considers how a character’s morality fits into the story, it’s rather easy to see the reasons why the character themselves are important. However, Matsuda doesn’t seem to be remarkable in the same way the other characters of Death Note are.
In fact, thinking about Matsuda made me realize his importance seems to rely mostly on the fact he is actually isn’t remarkable at all.
Matsuda doesn’t hold strong enough convictions to make his moral actions interesting on their own, and that’s what makes his moral actions unique in this context. He is there to show us the difference between the characters involved in the Kira case (the genius detectives as well as the more experienced law enforcement) and the people affected by the Kira case (regular people such as Light’s family).
As a police officer, Matsuda still lacks the resolve the older investigators have, and that, which appears to make him simple-minded or even stupid sounding, puts this character in the position of judging Kira not as law enforcement but as just another person who can see the criminality rates dropping.
That is not to say, in any way, that Matsuda isn’t willing to sacrifice himself. The values Matsuda holds true and that motivate him towards acts of selflessness are not motivated by the greater good, so to say. If we look closely, he acts kindly and in a caring manner not as a police officer but as a friend or an empathetic acquaintance. Matsuda’s acts of bravery don’t hold the same meaning as Soichiro’s or Aizawa’s because their reasoning is presented to us as something done because that’s the right thing to do despite their internal conflicts while Matsuda’s actions are shown to be motivated by a desire to do good but an inability to detach himself from his personal desires and conclusions. For example, to show the contrast between Matsuda and the other investigators, we can take Ide as an example.
Ide disagrees with L and walks away from the investigation, he is still unable to walk away from the case itself as that would be turning a blind eye into a situation considered morally wrong. Ide couldn’t bring himself to choose neutrality when he was faced with the consequences of letting Kira run free; walking away from the investigation becomes a shameful act. When Matsuda is faced with doubts regarding Near as L, he isn’t ready to choose his convictions over his work even when he seems convinced Near did terrible things to be able to end the Kira case.
By the end of the manga, Matsuda has acquired some experience and that’s enough to make him doubt Near’s intentions. Nevertheless, Matsuda is, still, only beginning to abandon the path of an ordinary person and truly becoming an investigator. As he chooses to put aside his worries about Near, he also shows us there is a transition to be made from an ordinary person who is meant to be protect by the law and whose concerns can be perceived as personal ones, an actual investigator who can understand doing the right thing requires more than a good heart.