Matsuda was never going to be anyone's hero. He wasn't anything special. He was never as smart as his parents wanted; his looks were merely average, eminently forgettable in so many ways. He tried his best, but he knew he'd never be more than an extra, a minor role at best. The starring role wasn't for him.
But he wanted it to be. Which was why he was so impressed with Light Yagami. Here was a boy who was everything Matsuda had always wanted to be: brilliant, good-looking, loved by everyone. Including Matsuda. He was kind, too, and didn't seem to dismiss him, like everyone else did. Like L did, not that Matsuda didn't know he deserved it. It wasn't like he didn't know he was dumb, after all; he just couldn't help it.
But with Light, things were different. He dared to think they were friends, dared to hope some of his glory would rub off on him. He flirted with his sister as the closest he could get to him, wanting to tie them closer together, knowing far too well that the likes of Light Yagami was not for him, except in dreams he would never admit to. And he waited for it to end. Sooner or later, he would tire of Matsuda and start treating him with contempt; they always did, the bright lights he so admired. He was drawn to their shining like a moth to light, but it always hurt him in the end.
This ending was unexpected, that's all. Never before had his idol fallen off his pedestal to shatter at his feet. Never had he seen so clearly that it had all been a lie. Matsuda wasn't smart, but he didn't like being made a fool of, not like this. The boy he thought was his friend was revealed to be his enemy, a monster, and as he raised the gun and pulled the trigger, he thought he was finally a hero. Just the ordinary guy, but the one who slayed the demon disguised as one of them when all the others hesitated.
But even that was taken away from him, the final blow not his. As he looked at the man who had once been his friend pleading for his life, he already felt regret. Even dead, even knowing that he had betrayed them all and always intended to, he was still the hero that was all Matsuda had ever wanted to be.