Solving a mystery because it isn't.

In chemistry in school at the moment, we're doing forensic science. And as part of this module, we have to "Solve a murder". A businessman has been shot at point blank range, there is some evidence around the room he was shot it.
We were given 5 suspects:

The son, the daughter, the housemaid, the gardener, and the handyman:

The son; Gave the statement that him and his father got on well; had blood stains on his clothes, said he helped his dad with a plaster after his dad cut himself.

The daughter: Gave the statement that her and her father didn't really see eye to eye, but there was respect. She has blood stains, said she was cutting something for her art (she is an artist)

The housemaid; Gave the statement that she thought son and father were fighting a lot, and was busy in kitchen and went down to ask if he wanted tea when she found him. She had blood stains, said she cut herself while chopping vegetables.

The gardener; Gave the statement that he was out spreading fertilizer when it happened; had blood stains, said he cut himself while cutting open bag of fertilizer.

The handyman; Gave the statement that he got on well with man, and didn't have any blood stains on clothes

Now, there were numerous test (ie. chemistry experiments) that we could do with evidence. One was to compare checks on Plaster, Fertilizer, and Clay (from sister) found in room.
BUT
Or chemistry teacher told us that we'd need to do more than the one test to know who it was.
So that test must have not shown any to be guilty, meaning that the son, the gardener, and the sister are free.

So we have the handyman and the housemaid left. Now, the housemaid said that the son and the father were always arguing, but the son, who we know is clear, said they got along well. There was very little other reason for the housemaid to say this, so I'm 90% sure that the housemaid is the murderer.

And the only reason it was possible to deduce this was because someone already knew the outcome, and slipped what she thought was useless information to us, when in fact it solved the case for us.

In a related note, I freakin love transition year!