In many ways, I am surprised that Willie even survived that first encounter with cousin Barnabas. Can you imagine how hungry he must have been after 170 odd years trapped in that coffin, not to mention, stark staring, time for a rubber room insane? Trapped not only with no way to get out but with a crucifix on the inside of the lid. Exquisite torture. Joshua may have not been able to bear destroy his own son, but after his son begged his father to end his torment, instead he made certain Barnabas suffered terribly. All he truly cared about was getting the problem off his hands, let someone else deal with after he's gone. Reprehensible.
Any ideas how he did survive with even a shred of sanity? Perhaps, slurping on Willie allowed him to regain enough composure to stop before killing his only (at that point) contact with the outside world?