We now know from the Marchesa that the murder took place within the garden, and the locking and unlocking of the door was an accident which looked like a clue....

But, if you witnessed the murder, and wished to retire without raising an alarm, or denouncing that unhappy lady, I ask myself why did you not open the garden door from within--the key was in the lock, I saw it--and pass out on to the high road.

Why did you, instead, try so hard to escape over the wall behind the ilexes that you tore your hands on the cut glass on the top?
I found the place next day.

There was blood on it.
When you were struggling to escape over that wall you were not anxious to take the Marchesa's guilt upon yourself.

When you were hiding behind the screen in the Duchess' apartment you were not--_at that moment_--very determined to shield the Marchesa from the consequences of her deed.