Victor Hugo has been able to show us the good side of a convict, a prostitute, and even a monomaniacal police officer, not to mention the countless urchins swarming the streets of Paris. Is there anyone he can't redeem?
This guy—battlefield thief, crooked innkeeper, con artist, and all-around crook—is the ultimate example of selfishness, greed, and cruelty. The narrator even tells us when we first meet Thénardier that we should be wary of men like him: "There are men whom we instantly mistrust, sensing the void that encloses them. They are uneasy at their back and threatening in front" (18.104.22.168). In other words, this dude is bad news, and not just because prison has twisted him up a little. (Whenever you see the word "void" used to describe someone, you probably want to back away slowly.)
As cruel as Thénardier is on the inside, he also looks the part on the outside. As the narrator tells us, "Thénardier was a small, skinny, sallow-faced man, bony, angular and puny, who looked ill but enjoyed excellent health – that was where his deceptiveness began" (22.214.171.124). Everything about this guy is calculated to serve his self-interest at all times.
Need some examples?
He lies to Fantine, saying that Cosette needs medicines and doctors, essentially forcing Fantine into selling off parts of her body to meet his demands for money.
He throws his sons on the streets because he can't be bothered with feeding them.
He tries to extort massive sums of money from Valjean.
He robs bodies after battles and even the student rebellion.
Need we say more? Well, one more thing. By the end of the book, Thénardier gets money from Marius to start a fresh life in the New World. You can almost see this as parallel to Valjean's fresh start at the beginning of the book: a wealthy benefactor gives a miscreant the money and motivation to start an entirely new life. But (mercifully) Hugo never wrote Les Mis: The Redemption of Thénardier. Instead, we're told that "Thénardier was incurable. He used the money to go into the slave-trade" (126.96.36.199).
No matter how many chances you give this guy, it looks like his selfishness and cruelty are beyond curing.