At some point after the encounter with Kaidou, Kurozumi Orochi offered Killer a chance to save his captain by giving him a SMILE fruit.
The fruit turned Killer into a Pleasure since it was a defective SIMLE. So Killer failed to gain any animal traits after eating the defective SMILE. And due to its side effects, he now always expresses emotions of pleasure, smiling and laughing all the time regardless of his true feelings, even when taking harm.
In the new SBS Volume 101, Oda revealed that if Killer’s SMILE had been successful, it would have been an elephant-type which would have turned almost his entire body into an elephant, with his head being at the tip of the elephant’s trunk and his arms protruding from the elephant’s sides.