It say that Wald's poison is so vicious, then why J touch his hand in his bedroom cause Wald to extinguished?

It’s a cycle. Wald wanted Jonathan to surpass him so he could die. He did exactly the same thing to Jonathan that Jonathan did to Mako. Jonathan was in love with Wald and Wald took himself, Jonathan’s heart, and his reason for existing away from Jonathan when he extinguished himself. Jonathan did the same thing to Mako, and Mako will do the same thing to the next demon. It’s a cycle that probably started even before Wald.