I'm going to go out on a limb and say probably badly written.
Basic premice is: "This character has been groomed since birth to do [thing], but now growing up they don't want to do [thing] anymore."
That's a common, normal premice. Cf every single "I am prince but don't want to become king" story.
However your explanation doesn't ring human, it rings like a puppet, he seems more or less ambivalent to the death of another by his hand. As you describe he doesn't want to do it through laziness? No internal or external motivation, just can't be bothered.
Usually when people have been groomed into believing something is important for decades, they need a reason (o'r usually a handful of reasons) to "fall out of the faith" so to speak, and these reasons almost always come from emotion that cannot be ignored, the feeling that something is not right, or something that forces doubt about a fundamental principle into the mind. Remember you've set up that this guy's whole world view was shaped by this mission, his purpose in life was this, and now it's not. That is a massive and sometimes traumatic shift in someone's psyche, and you've not looked into that because that character was always supposed to not want to kill the big bad in your mind.
Think about when you're forced to do something you don't want to do, worse something you think is actively wrong, you're going to feel some type of way about it, usually negatively. Resentful against those forcing you to do it? Terrified, distraught, depressed, avoidant?
There's a distinct lack of emotion and humanity in your description of your character's goals and actions here. If your writing in the story is anything like this, the motivations are strange because they lack emotional recoil to big life events and decisions, and if they don't effect the character, they won't effect the reader.