Thinking that there was finally a squanderer, if he didn't rip him/her off, he would feel sorry for himself. (Probably)
Squanderer might need to be replaced... I've seen this quite frequently in novels but the translators never use squanderer lol
He thought in his heart, it wasn't easy to get a fat head, if he didn't pluck a lot, it would be doing himself an injustice. (Literal translation) Cleaned up: He thought in his heart, it wasn't easy to find a sucker, if he didn't rip him off, it would be doing himself an injustice.
He thought, "Finally a great fool has taken bait, if I don't rip him off then it serves no justice to myself."