Doing a quick rundown on all the possible suspects of Umineko and whether I think that's possible or not. I'll probably crosspost this to the blog.

Krauss- I'm split on this. In one view, he already is set up to inherit the money and has no reason to kill the family, in the other, it may be possible to inherit the money.
Natushi-She definitely has motive, having no blood ties to the family, but after her role in Episode 1, I think she's a huge red herring.
Jessica-Doesn't have it in her.
Eva- Thanks to a split personality, I might believe this. However, as badly as Eva wants to inherit the money, I think her bark is worse then her bite. Especially as she's presented as the murderer in episode 3. However, following on of my theories (see George and Shannon), I'd believe that she snapped after George's and her husband's death and went insane.
Hideyoshi-Doesn't have in him.
George- I strongly believe that he and Shannon have had something to do with the murders, my guess is in episode 4. Their relationship is just so boring. Why? Simple. I WILL BE WITH SHANNON AT ANY COST, LOL. Further evidence? He is remarkably calm over his father's death. My guess is his death in this arch is a suicide, which sets off Eva.
Rudolf- Has the skill, and wants the money. I kinda doubt he has it in, but I'm not sure.
Kyrie- Is a very likely suspect. I'm willing to bet she had something to do with the first arch. Kyrie has mostly been a background character, but we finally see in Episode 3 that she has a lot of anger, and not afraid to act on it. She also at least has motive for killing Battler, and possibly her own husband.
Rosa- Needs the money, and has a temper. I think she may be a bit of a red herring BUT I do believe she had something to do with the murders in the second arc. Not without aid though.
Maria-While she's messed up, I don't think she's that messed up. She just wants her mommy to love her and to be a witch.
Genji-I think all the servents except Shannon were in on a plot in Episode 2, maybe with or without Rosa. However, a dispute emerged between everyone else and Kanon leading to a bloody battle to which Shannon found out about, and had to be killed. Alternatively, Kanon could have escaped then killed Shannon and George.
Nanjo-Has motive in that he needs the money. See Genji.
Ghoda-See Genji
Kanon-See Genji
Shannon-Continuing from George, Shannon is the one killed by accident on the theory Battler presents. However, it's possible she doesn't die right away/fakes her death somehow, but later she does somehow die. So either George kills her, George and Shannon are locked into a suicide pact, or she dies and then George kills himself.
Kumasawa- She kills everyone for being prettier finding other people prettier then her. In truth? See Genji.
Kinzo-He's waaaay too into that occult stuff to resort to actual murder. He'd do some hokey spell to try to kill everyone.
Battler- Doesn't have it in him.
Anyone else-It's one of the orginal 18
Mackeral- Damn mackeral.
WHAT ARE YOU TALKING ABOUT, WITCHES DID IT?- And aliens. Really, I wish it was this, but it can't be that easy. ^_^