When asked why he did it, Lucio was going to answer after a bit of thinking. Because he himself did not know the answer. Then, after thinking for a long time, he would finally choose from one of two answers. One of them would be 'I just moved my body without my knowledge', and the other one... It would be 'to repay the debt of the heart.' Lucio thought it would probably be one of the two.
To be honest, the two answers above barely fit as reasons, and they were not the exact reason. Even he couldn't explain and know the exact reason for his actions.
Obviously, he always had a feeling of being sorry towards her. To leave her alone when she was his real wife, and to favor Rosemond the mistress, and the crime Rosemond committed when she had the meeting with the envoys' wives, all of it. But for both reasons, there was nothing he could do about it anymore.