If you trained a monkey to kill, and it did so, then would you be
guilty of murder, or another crime?

Certainly. It would be no different than commanding a pack of trained
dogs to kill someone or putting a poisonous snake in someone's shoe.

tutuzdad-ga

I don't know if the question was serious, or just tongue-in-cheek,
but still -- is a monkey strong enough to kill someone?  Maybe it
could trip someone going downstairs.

And how would you prove the crime?  The monkey is unlikely to confess,
and even less likely to give testimony in court (although he might
point at the accused).

Blood evidence might help. (You know, the "rhesus factor".)

Murder, just like if you used a knife.  A monkey has no legally
recognizable independent capacity for intent.  Nasty monkey business
if you ask me...

Where can I buy one of these trained homocidal monkeys ?  Just asking...

>>>is a monkey strong enough to kill someone?  

Uh, yeah...if you call tearing someone to shreds strong enough to kill someone.

Chimpanzee attack leaves man in critical condition
http://www.kget.com/news/local/story.aspx?content_id=4CAC625F-2D42-47D1-BBFA-982FF1C9DABC

Chimpanzee attack probed - Sanctuary visitor severely mauled
http://www.boston.com/news/nation/articles/2005/03/05/chimpanzee_attack_probed/

Chimpanzee Attack Doesn't Surprise Experts
'Experts Not Surprised by Calif. Chimp Attack, Say Animals Are
Aggressors With Great Strength'
http://abcnews.go.com/US/wireStory?id=553553&CMP=OTC-RSSFeeds0312

You better think twice before you call a chimpanzee a monkey.
He might "go ape" on you!

When ape-human hybrids start to appear, the legal status of such
anihumans would be quite interesting...

Jason,
Before you get him "fully trained", you will have probably been picked
up on some other charge, maybe assault.
Incidentally, the Bible deals with this problem and that of owners of
overly aggressive dogs.  In Leviticus (I believe), the law says that
the owner of a bull that kills someone is not held responsible  - or
only to a lesser degree - if the bull was not known to be aggressive. 
But if it was, then the owner could suffer the death penalty.
Myoarin

You should read Edgar Allen Poe's "The Murder in the Rue Morgue",
or maybe you have ...