You arrive at the scene mere seconds too late.  Alice and Bruce,
half-empty syringes in hand, stand above the apparently dead Cecil,
who himself clutches a needle.  You were afraid something like this
would happen...  But just then Cecil's pale lips begin to quiver, and
in a painfully forced whisper he mutters, "Well *this* is unexpected.
The truth serum must partially counter-act the poison..."  You implore
as to Cecil's wellness, but he assures you that he is indeed dead,
just able to talk for the time being.

Your thoughts quickly turn from the perplexing status of your friend
to determining which of Alice and Bruce has proven to be an enemy.
You take account of the situation: Alice's syringe is labeled as Serum
X, Bruce's as Serum Y, and Cecil's as Serum Z; all three hold
similar-looking concoctions, each glowing a malevolent shade of
electric green.  The moment you think to ask anything, Alice blurts
out, "It was Bruce!  Bruce stuck Cecil and pumped him full of the
highly poisonous Serum Y!"  Immediately, Bruce retorts with, "Ha!
Cecil injected his mixture into himself, just before he jabbed you
with his needle, as well!"  Their brief retellings don't quite seem to
add up, and you realize that, as potential murder suspects, neither
Alice nor Bruce have any incentive to tell the truth, unless...

Cecil tries, but utterly fails to clear his throat.  "It all happened
so fast, I can't quite remember all the details.  I am sure, however,
that Alice managed to get Bruce with her syringe, too.  We might have
to break out the chemistry set to solve this one... well, for now,
know that any green serums that we can access that leave their
recipients alive are truth serums.  Anyone injected with them cannot
lie, though they can withhold the full truth."  You assume you can
trust your friend--after all, they always say that the dead don't
lie--but you're not so sure about the others.

Who is the murderer?  Which serum is what?  Who is telling the truth?