Saul Goodman was Mike Ehrmentraut's attorney, Jesse Pinkman's attorney and Walter White's attorney. He knew that the DEA would come looking for him regarding what he knew about Walt's Blue Meth empire especially knowledge about the money and the murders connected to it. Saul Goodman knew that Hewell and Kuby had gone missing so he knew that someone was either coming after those people connected with Walt to arrest them or coming after them to kill them before they could talk. He knew that Walt was involved in an international drug ring and had ties to Neo-Nazi hitmen and he didn't want to stick around long enough to find out if they wanted to have a word with him. Jesse had disappeared and he knew everything. After Hank died, Saul knew that the DEA was going to be coming at Walt with everything they have. He's a lawyer - he did the math. IF he gave up everything he knew about Walt and cut a deal with the DEA, he would still never be safe from Lydia, the Neo-Nazis or Walt himself. Whereas if he leaves town, he doesn't have to cut a deal with anybody and he can probably stay alive. Anybody else who found himself in Saul Goodman's shoes would have done exactly the same thing.
From yahoo