A golem core shields its soul from evil
The reason that a golem core can only be entered by the willing, has to do with its construction, it was constructed with that one person in mind, and the magic pulls them immediately towards the golem.
“But you can bring <PERSON> back.” <PERSON> fixed his eyes on the witch intently.
“I can,” she sniffed, “but not without... issue.”
“Issue?! You created the Shrine Protector, there were no issues!”
“Yes, that was <PERSON>, and we molded the Shrine Protector to his soul while he was still alive. They have to be a great fit, you know, the body and the soul, otherwise they just slide off. They are always being drawn back to the Veil. So you would need to bring me <PERSON>’s body—nothing else will fit her soul proper, now. Unless she was already in a golem and didn’t tell you. So I’ll need her body, head intact, and not too decayed! Healing magic on the dead is... clumsy.”
And then there is the evil part. Either the evil is part of this world and it “rolls off” of our bodies normally but she has lost that defense mechanism, or the good is being constantly produced by the soul but now it cannot stay close by:
“Okay, I’ll do it! We need her! I... I need her.” <PERSON> hid his tears, but she could see his eyes were misty. This was not just about the castle. He started for the door. She grabbed his arm.
“You need to understand,” she said. “<PERSON> will not be the same.”
“Not the same, how?”
“Once we pull her soul back from the Veil, and bind it back to the body, it keeps getting drawn back. All of its light gets sucked that way. You may find her darker. Angrier. 624
Crueler. No emotional control, acting like the world exists just for her. All of that light which your soul can keep close, all of that is constantly returning to the Veil.”
“So, she acts, I guess, like people act when they are hurt? Am I sentencing her to a life of pain?”
“Well, yes and no. Pain snuffs out that light too, but from what I know of the folks who have ‘come back’ this way... it doesn’t hurt, as far as I can tell, but they just aren’t the same. Death-touched, is what the ancients used to call it—they forbade this practice outright, don’t you know. She’ll be touched by death.” The witch was clearly running out of words to describe it.
“But she will still know the way through the caves to get into the castle?”
“Hard to say. Probably? But it is very likely that her love for you will be gone. You may have to persuade her some other way, some appeal to her more... selfish interests. Power. Lust. Fear. Take your pick. She’s going to be half-animal now. Death-touched. She’s not going to want to depose the Prince just ’cause it’s the right thing to do. That light in her is constantly drawn back to the Veil.” 417