spoiler ish
actually he knew that HE was gonna do the operations/surgery on his brother and he couldnt bear to do it to his own brother due too the HIGH and VERY POSSIBLE risk of sunwoo's mpreg project. he has 80% rate of dying and his brother doesn't want his own brother to die in his hands + his hands will be shaking throughout the whole surgery since hes performing it on his own spoilt brother.
he ran away cuz he doesnt wanna see sunwoo puppy fac-ing him (force him) to do a surgery on him.
yes hes a trashy brother but yeah imagine having to perform a surgery on your own family
i'd honestly probably run away too , i could never have a family member's life on my hands, that's crazy
How can you abandon your little brother especially when he's carrying