The road to being happy is simple to Cato: kill everyone.
Alliances were forged—only in convenience—to be broken later. Every weakness displayed remembered for when it could be used to demolish that person. Never sleeping without his sword clutched in his hand, never sleeping less than prepared to rouse and kill them all.
Cato was there to survive, to do well by his family, his training.
Then he could go home and be happy again.
(How strange it is, then, that his last thought is how happy he is he'll never have to live with any of this.)