Like I said, his memory of him being a Chaser was lost, stolen, taken whatever! When he tried to kill the other Chasers, he lost to them and lost his memory. Even if he was battling, he didn't have any memory of being a Chaser. And how do you know the Dark Keyblade wasn't his? You don't and I don't. It's called a theory. It wasn't Riku's Keyblade, he didn't have one at the time. His Keyblade is the Way to the Dawn. Riku didn't use the Dark Keyblade. Xehanort's Heartless used it, only through Riku's body.
Thought:
Ok, for the time being, Riku has control over Xehaonrt's Heartless, right?