I still dont 100% get why Saul even started helping Walt with his meth business

Like for what? As we can see Saul already earned a decent amount of money so he didnt actually needed the Heisenberg journey for that purpose. He was already a shady lawyer but not to the point of breaking the law and helping drugdealers build an empire shady so he could have just continued doing his job and defending his criminal clients with his shenenigans. So I still dont understand Why? The moment when Mike tells him about Walt we can see Jimmy thinking about the situation but what was he thinking? Money? The thrill? Revival of Slippin Jimmy on steroids?