Why does Philip Lombard think that Mr. Rogers couldn't be the killer?

Lonbard and Armstrong both believe that Mr. Roger's is scared out of his senses. Thus, they do not believe that he's the killer.

"Ah!" Armstrong breathed a sigh of relief. "You agree. But you see what it involves? Rogers swore that there was no one but ourselves and he and his wife on the island."

"Rogers is wrong! Or possibly Rogers is lying!"

Armstrong shook his head.

"I don't think he's lying. The man's scared. He's scared nearly out of his senses."

Philip Lombard nodded.