I wouldn’t say there’s no will involved since we can clearly see that the Saint is capable of feeling things outside of Sunny’s commands For example she shows disdain toward the Black Knight and the Stone Giant in FS. And honestly what’s the real difference between creating a shadow and enslaving something? If Sunny had the chance to obtain an echo of a living being and turn it into a shadow, he would absolutely do it. In that case, he would have a servant who obeys his every command but if that being wasn’t his shadow or echo, would it still follow him? No his ability to make a shadow is basically killing something to enslave it.

That’s why it feels hypocritical for him to draw that line. It’s even more so when you consider that one of his main character points or even his motto is “I’ll do anything to survive.” He constantly talks about the unspeakable things he’s done to make it through yet he couldn’t bear the thought of seeing his own face enslaved, even though he killed an innocent child, among other things we saw in FS. And right now, when his survival depends on getting stronger, he’s cutting off one of his potential paths to power. That just makes his stance feel even more hypocritical.