Wasn't Mina the maid that ratted him out on several occasions, because of her greet for sweet? So getting revenge on her with a little prank (in this case a rather big one) on her is justifiable. I can't really hate him for that. (Making her believe she'll die any moment went a bit far, but remember: He gets absolutly plundered by the others and she is also totally fine with it.)
Of course his sis just swooped in and turned on the light, told Mina its just her brother, MC also got his chance to say it's just a prank, all got a good laugh and went to bed, not. She escalated the situation, swang her sword so strongly it beheaded a plushy (beheading a plush-toy with a wooden sword is quite a feat), and stirred hell up. Then again Mina feared for her live and it happened infront of Sis's bedroom, so she is in the clear I guess, eventhough she noticed from the beginning it was Al. Why is it always all the way battlevalkyrie with her?)
I'd totally excuse MC for scaring the maid shitless, if it weren't for MC memories of another life. I totally see him doing even crasier shit without those, as going too far seems to be the norm in that family
#JusticeForMrKnight