I think that the "prince" is twisted to freak out (like that) when he found out that Yamaguchi was having sex with sensei. If he tried to "save" sensei (the right way), then I might ave agreed with you. But, now he raped sensei and Yamaguchi had to save him (sadly not in time). Now, I can only cheer for (let) Yamaguchi have sensei because that "prince" is now a psychotic yandere *sshole. (/TДT)/
Honestly, if Yamaguchi never raped sensei, the young master might not be so crazy. And then Sensei and Young Master might have figured out each other's feelings, and they could've been a cute fluffy couple. But NOOOOO, everyone has to be going and raping sensei. Wtf (╯°Д °)╯╧╧