Judging by the information given,
The murderer is the Cook.
First of,
Bodyguard - only ways to get in to kill is by window in
kitchen,
or by front door during the sound on the roof checks.
but he won't know if the window is going to be still opened at
night,
unless he is in cahoots with the cook. but then he won't know
the situation in the house, thus cannot plan who will see
him.
since he's a Bodyguard, he'll probably ownz uncle John (not pretty
good fight).
8 year old son - since uncle John put up a pretty good fight,
it wouldn't be his 8 year old son.
Nanny - since she is accompanying the son, the son would have
noticed something strange about her, plus she's a female.
see above pretty good fight.
Wife - female, looks like the best person for the job, but a
murderer
will create impression that its not him/her, plus she's a
female.
see above pretty good fight.
(high chance she's in cahoots with the Cook - reason for the
murder)
Butler - he could be lying about having a diahaerra, but since he
keeps
going to toilet (assuming toilet is in the first floor) he has no
chance to
commit crime because the maid is at the up resided staircase or
with him checking the roof.
Maid - only person "guarding" the staircase, but the only strange
thing
happen in the night was the sound on the roof, and she is one of
the
two that went to check, so its not her plus she's a female.
see above pretty good fight.
Cook -
open window in the kitchen is likely by him, reason being:
1) he didn't expect the maid to be there, so he went to the
kitchen,
thru the window, makes a sound on the roof (or release the cat to
the roof).
2) makes it looks like somebody from outside did it.
3) creates an impression the Bodyguard would have escaped from
there
it is most likely him because:
1) he's on the first floor (able to access the kitchen)
2) he has the ability to let the butler get a severe
diarrhea,
able to make the butler will be visiting the toilet often
3) he's a guy and likely to put up a pretty good fight with uncle
John
4) only person to ensure that the kitchen window would be
opened
Hope i'm right! 
