Yoonseul has the right to be possessive towards kang joon especially when Dongha doesn't know when to stop. He's trying to send a hint to Dongha to stop coming after his man. Yoomseul's possessiveness isn't even too extreme, he wants to punch the fck out of Dongha but choose to stay calm for Joonie. If my boyfriend has a co-worker like that i'd be mad and become possessive too.