Yeah, I could really see that at least with Alucard and Soma. Also, in SotN, there could have been more than the librarian. Based on what Death says in the first meeting, Alucard was with them at one point in the past. That being said, there could be individuals in the castle who now him and may want to help. If I remember correctly in the CoD manga, the castle was a haven for individuals how hated God. So, if that is true, not all of the castles inhabitants may be evil or at least nonviolent. Plus, money is a source of power and for a price individuals will likely help out somewhat. After all, everyone has a price (though it differs from person to person). Hell, there could be some what if situations. For example, the battle with Olrox in SotN. There could have been a dialogue in which Olrox asks Alucard what he is doing. At which point, you could choose an answer. And, depending on how you answer the question(s), you may be able to avoid the battle and get the relic in the room beyond. Let's say that Olrox indeed asks the question, one answer could be the following: "I've come to organize a coup and take my father's place as ruler of this castle w/ you as my second in command and Death as your subordinate." At which point, he can say: "If I can be above Death, then I will assist you. Don't you dare fail." With that, a new shop with different stuff than the library shop could unlocked. Of course, if you give a different answer and Olrox fights you, the shop would not unlock. It would be a one shot event.
If something like this is added, I think that it would add some real depth to metroidvanias that start characters with dark powers like Alucard and Soma.