I think he didn't have a choice. My gut is that Ryoko crawled above Dexter's room. She tries to shoot him (either because she wants to kill him or because Dexter attacked first); Spiral hears a shot. After a fight, Dexter manages to stab Ryoko. Since it's late into the night (Ryoko was coming back from the second floor, so it should have been after 3:00 at the earliest), Dexter doesn't want to go out and risk being spotted.
However, he's afraid that we'll do another room search the following day. Which is why he hides the body in the laundry room. And while he's doing that, he hears Biscuit and I coming along, hurries to the second pad and tries to input the password to have a reason for being outside.
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When you have nothing to say, quote yourself ~ Solfadore