i hate where this is going..and the boss still doesn't give a VALID reason to strangle the...

I think he didn't want to his dad to marry to his mom so he strangled cause they would break things off?

THIS!! I agree! I'm thinking Sihyeon started having feelings for his little brother, so when Dajeong mentioned their parents marrying, it elicited that overly strong reaction within him bc even though he seemed happy about the Parents' relationship on the outside, on the inside--bc of his improper feelings--it was probably eating him up.