Why did
David, pretending to be Walter
let the Alien out on board the ship?
He already had the crew fooled, so all he had to do was to wait for them to go into hypersleep and then he could conduct as many experiments as he wanted.
Letting the Alien out was very risky: he lost three perfectly good bodies (the one with his cheek burned-out with acid and the couple in the shower) and the alien could also possibly kill all the colonists or even destroy the whole space ship.