In"The Ransom of Red Chief," why does Bill say that the kidnapping plan struck them at a moment when they weren't thinking clearly?

He says this because, as the story later turns out, they regret those plans and it all turns out just the opposite of how they intended.  He specifically states, "It was, as Bill afterward expressed it, 'during a moment of temporary mental apparition'; but we didn't find that out till later."  So, they had what they thought was a sudden light bulb of a great idea, but it doesn't end up being a very good idea at all.  Their charge is simply awful to be around, and Bill has to put up with all sorts of mischief and degradations at his hands before they finally decided to pay the kid's father to take him back.  Not a very successful kidnapping scheme at all.

It also helps to understand that O. Henry is a funny writer; he often has a sarcastic tone to his stories, and Bill is being funny and a bit sarcastic when he said this.  The entire story is very funny, and Bill's sarcasm throughout aids in that humor.  So knowing O. Henry's writing style, and the rather wry character of Bill help us to understand that the comment was made in sarcastic hindsight.