Basically (I don't think I'm the first one to propose this theory at all), it's that he did use the unexpected human energy he got in the eye-healing to make himself into a bomb right from the point when it happened. Like, he didn't know it would happen (he does look surprised in that scene) but he realizes immediately how to use it, and does.
When he says "exchange all the energy in his body" I don't think he's being completely honest. I think he's letting out as much dark energy as he can so he can nurture the light energy to make a more effective bomb. He knows he's going to have to counteract the dark energy in Ye Zun so he's going to need a higher light/dark ratio in his own. It's why he's so incredibly weak there at the end. He's not been exchanging all the energy in his body so much as he's been letting out the dark energy to make room for more light energy. (eta: I also think this isn't the only time he cuts himself, it's just the only time he does it where we/ZYL see it)
So yes, he's been building himself into a bomb since the eye-healing. It's why "worth it" comes out of him so readily--because he doesn't think he is worthy of ZYL (for various reasons, probably including keeping his brother locked up, letting Dixing fall into such disrepair, all the other stuff that isn't his fault but he takes responsibility for). It's why he smiles at ZYL as he's dying on the floor in the palace. He thinks martyrdom is what will make him finally worth it.
Do I like the theory? No. Does it seem highly probable? Yes.
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When he says "exchange all the energy in his body" I don't think he's being completely honest. I think he's letting out as much dark energy as he can so he can nurture the light energy to make a more effective bomb. He knows he's going to have to counteract the dark energy in Ye Zun so he's going to need a higher light/dark ratio in his own. It's why he's so incredibly weak there at the end. He's not been exchanging all the energy in his body so much as he's been letting out the dark energy to make room for more light energy. (eta: I also think this isn't the only time he cuts himself, it's just the only time he does it where we/ZYL see it)
So yes, he's been building himself into a bomb since the eye-healing. It's why "worth it" comes out of him so readily--because he doesn't think he is worthy of ZYL (for various reasons, probably including keeping his brother locked up, letting Dixing fall into such disrepair, all the other stuff that isn't his fault but he takes responsibility for). It's why he smiles at ZYL as he's dying on the floor in the palace. He thinks martyrdom is what will make him finally worth it.
Do I like the theory? No. Does it seem highly probable? Yes.