Yeah that's where he was wrong ..very wrong..that's why they separated for 5 yrs..ss said no ke I don't know in this while which all moves were calculated & which were sincere ..although she didn't doubt his feelings but he was using her at times in his grand scheme of things which she couldn't bear..
But in his defence ..
He knew very well what he's been planning for so long , it might end up real bad..but then his only hope was that he could find evidence against Ling yi and implicate him lawfully .....he was constantly working towards it but when peng Kang died , that last bit of hope was lost ..n that's when he decided to end it all..
That's the one and only loose end of the story ..the evidence which even he couldn't put his hands on , niao niao found it so easily and exonerated him 😁 all of it looked very easy but I don't think it was..just bec we saw the full story & that emperor helped him later , heard him out , he was saved frm the cliff , they also found evidence to save him , it looked that way ke if he told her earlier on , and they married first, things would work out one way or the other ..but it really wasn't certain ..he didn't plan out that one thing ke niao knows it all n she would be standing right behind him , ready to face the consequences..he actually planned to die
He wouldn't marry her before he carried out his revenge..that's the reason their marriage was delayed & everything was delayed ..like betrothal gifts and all..it was a fact ke if they married & his plan goes south , niao niao and her whole family would perish with him ..he witnessed his clan's death , he couldn't let her be the reason for the cheng family's death..that one time she risked her life to save them , they were her dear family..how could she see them getting executed bec of her and her husband, incase they married ?
What broke her was he abandoned her on the cliff n said their fated relationship has ended ..n when he said he doesn't regret leaving her n die , she ended it with him
I felt scared and broken when he pushed her front and jumped backwards in the cliff.. Like goosebumps.. NN asked him till the very last till before she left the palace if he wants to say something to her.. I felt so bad when her dad said to ling buyi that half built coffin for NN is still in their garden after he abandoned.. I wish this seperation could have been handled in a different way.. But still nevertheless this show is masterpiece itself..
). That's where he felt the regret. Regret for not choosing the love & happiness that was waiting for him. 
So contrived.. they made it look like it was so easy ..otherwise the tiger tally theft was alone to execute him I guess it amounted to treason ?
Bahut hi mazedar script likha tha..that dialogue no..find something better than gossiping about women's marriages 🤣

Ofcourse she didn't think about him romantically. He was destined to be the younger version of his wolf uncle