Why should it matter if he had some ulterior motive for going into this relationship

Because he was never informed of such motives! Yigon informed Taekang of his motives despite being told that it still sucks pretty much that someone is using you for all your worth without you being more the wiser lol

Plus it’s not like Yigon hid his insecurities and fears so it makes sense why he feels slighted by the fact that even if Taekang had an ulterior motive. It’s the optics of it all

It was an arrangement, wasnt it supposed to be strictly no-strings? This is besides the fact MC grew feelings for ML. There are bigger, worse glaring issues, like the lack of trust ML demonstrated over and over again ultimately pushing MC to deaths door. I just think its kinda silly to mull over something insignificant bcs many things can motivate people to so numerous things, feelings can grow despite of something like, being in a relationship to avoid a real marriage but what you cant comeback is lack of trust, complete abandonment over night bcs he just happen to see MC at his company lol Despite practically begging to not do this that his brother would kill him and that he needed proof of his almost death to finally believe him. Kinda silly, no

Because after Taekang decided to nullify their contract, Yigon has accepted that he was wrong to think Taekang has any feelings for him, so it surprised him to see that Taekang could act on his emotions–or, as Yigon said, his emotions are at the forefront just like he was when he liked him. It has nothing to do with which motive is more noble

I would say it's a matter of security. Going back to Tea-kang would make Yigon's brother very angry and If Tae-kang wanted Yigon back only for the sake of his deal with his grand-father then he could ditch Yigon at anytime when he find a replacement which would be a death sentence for Yigon.