I thought/understood the same about the morphine.
About the 'thirst', I think he's also talking about Taki (because he wants him but he doesn't want to hurt him like the last time)
i think that they are talking about some kind of drug in this case a liquid drug maybe heroin, i am sure is not morphine...
morphine is a pain killer that is very addictive. in a sense, its the quickest way to relieve pain but it has a bad side effect. it makes the user aggresive. in the case of "thirst" it might be he thirst for the "flower" that he's family has been looking for but have never successful at obtaining.
oh and his "thirst" for taki to give him the same love as he gives him.
Hey, is Klaus a morphine addict? it seems like it but im not quite sure? what does he mean when he says "thirst" anyway?