It was definitely a question by Tom in movie and book.

In book, Carlo's low level bookmaking job was the Family's way of "giving him a living" but unusually Carlo's job performance was reported directly to Vito/Tom. There's an incident in book where Carlo had made a mistake transcribing odds and cost the Family money. And this is said to have confirmed Vito's judgment of his son-in-law. Even before all this Connie is said to have disappointed Vito in her choice of a husband.

Getting back to movie I think that someone like Vito by training and birth just had an ability to "read" people quickly. He didn't like the vibe he got from Carlo and wanted to minimize his/Family's exposure to him. I guess they could have found a spot for him at the olive oil company but then that meant Vito would have to see him more often.
And Vito already had one idiot (Fredo) to see every day. wink

"When the snows fall and the white winds blow, the lone wolf dies but the pack survives."
Winter is Coming

Now this is the Law of the Jungle—as old and as true as the sky; And the wolf that shall keep it may prosper, but the wolf that shall break it must die.
As the creeper that girdles the tree-trunk, the Law runneth forward and back; For the strength of the Pack is the Wolf, and the strength of the Wolf is the Pack.