I've been watching a lot but I just added undead murder farce to the list. One of the worst names IMO but the show is actually quite good. It's a mystery show with good attention to detail and logic so far. A pleasant rarity. Theres also a solid wit to the dialogue. I'm genuinely unsure of who the killer is, but I do have a guess.
The most basic answer would be the blonde son. That is where the show is trying to point us and he did have his hands in his pockets as much as he could. I believe the more interesting solution would be the daughter with help from the maid. Logic would dictate one of the vampires would have had to have broken the lock as no way a human could do that, however, a vampire can't carry the stake. If the daughter broke the lock, had the maid carry the stake and holy water, then it works out.
Additionally the blonde son seems too incompetent. He was spotted by our half oni when he thought he was well hidden. Furthermore, the daughter made sure to sing loudly, covering up any sound and delaying people from checking on the mother. Additionally, the daughter twice interrupted aya's line of questioning, once by appearing in the room and once by leaving the dinner table. While the daughter appears to be a child, it is a vampire, it could very well be 100 years old and the show wants us to think she's too young and innocent for that.
I believe the brothers having only half an alibi is a red herring. It's where the viewers attention is supposed to be drawn. The answer to a good mystery however, is precisely where the author doesn't want you to look. It's very similar to slight of hand magic in that way. Well I guess slight of hand is just a type of mystery anyway.
The only other thing of note to me is that the butler was with him before he was a human ally and he tried to leave when shizuka was watching them. It's plausible that the butler would want things to go back to how they were before allying with the humans, but thats somewhat flimsy and really most of the people there do. Additionally, him having the key is a good way to throw someone off the trail if he was somehow able to break the lock. People would think he would have no need to break it with the key but thats really not the case unless he wants to incriminate himself.
So in conclusion, the weakest plausible answer is the boy, the medium quality answer is the butler, the high quality answer is the daughter and maid.
This show seems to follow a lot of the rules of mystery. I can suspect that next episode will have us going over suspects one at a time while making each one look guilty, however it's the one not shown till the end thats actually guilty. The thing is, in mystery, everyone should be hiding something. We can assume the brothers and butler were hiding something, just that it wasn't the murder.
What will upset me is if it's a plan from the "M" guy to send a hunter with the silver stake to enable the murder, trusting for basically no reason that godard would put the stake in his house instead of just disposing of it despite not having any plausible way to know anything about them. I foresee this possibility but IMO it would be bad writing.