Matsuda kun is sort of behind his team members, everybody was easily ignored and annoyed with the young officer. The image of Matsuda is that he's a little clumsy, he ended up being called "Matsuda Idiot!" Yet he seriously wanted help everybody. Matsuda felt himself an outsider, and wanted proof himself that he is a reliable man. At the end of the season, he dramatically proved himself as reliable he can be in front of his team members.
Even after Kira found out, Mikami contacted Takada and told her write down all their names. Before Kira found out it was fake, he claimed himself is winner. Near was waiting to see Kira's confident face turn to panic. He hadn't anywhere to turn. He was like a mouse chased by chat into a corner. He couldn't turn nor move. Kira finally admitted that he was Kira.
Matsuda couldn't understand why Light deceived own father. He was so furious, because Soichiro never had given up his hope that his son was not Kira. Matsuda asked Light what was meant by his father's death? He hoped that Light might regret killing so many people.
But Light's ambition was only one thing. He wanted create a new world and became a god of new world. That's all. Whoever got in Kira's way was punished by this new god. He was the only one who could cleanse this rotten world. He killed too many people, and forgot about the importance of human life. Light lost his principles, particularly that about not killing innocent people. His hope was remembering his father's last words that he was so happy to see lives expand. He had never known his own son deceived him until he died.
Light was quiet for a moment, and replied to him with coldness, in an unemotional voice, that his father was one of a weak human race, and part of the reason this world has sunken to rock bottom - humanity was rotten as could be. Because the crime rate is so high, they were overwhelmed and unable to control all these crimes. It was just a matter of the time before the police adapted their system to suit criminals, who would change their perspective and control the police. Rotten people only produce evil because of their deeds the world can only occur that way. As a result rottenness sprayed out across all the world.
Light continued saying that His father was a fool, because Light gave him a chance to use the Death Note. But Soichiro was cowardly and couldn't use it.
Kira looked at Matsuda with no emotion, just cold eyes. There was a mention that Light had been using his Death Note for six years, which meant Light had long since lost himself. Light underestimated the Death Note's power and the longer he used it, the more its power pulled him into the dark and took over his mind.
Remember Episode 9? When L was still alive, L showed up and gave Light a surprise at college? Light was so furious, because he got humiliated by L. Ryuk was seeing his outrage from behind, and in astonishment said, "Light was usually a quiet guy, but I've never seen his rage like this. It got into him!" Yet Light was just sneering about how weak his father was.
Again, Matsuda trusted Light; also Matsuda was fond of Light. Light was Matsuda's friend and he was also the last person to believe Light.
I think Matsuda made the right decision to take action. He connects with anyone, heart to heart. He was sort of clumsy, but that was his personality. I think his optimistic character had balanced with this dark story. He is one my favorite characters.