He was your friend. And that's all that mattered.
Mello was your friend, and he needed you, and that's what friends are for. And besides, you had nothing better to do.
"Why not?" You had said. And that was the end of it.
But, really, you couldn't give a flying fuck about Kira, or saving the world. The only thing that was keeping you here was the fact that Mello still needed you. And you were his friend, and he needed you. You had to stay, even if it meant risking your own life. But you still didn't really care about this whole thing-
Not really.
And maybe that's why you lost your life so easily. Maybe that's why you're laying on the ground, bleeding bleeding bleeding, dying dying dying, struggling for your last breath.
And even though you only have seconds to live, you still don't regret doing this.
You helped Mello when he needed you, and that's all that mattered, because he was your friend. And that's what friends are for.