How could you argue that Cassius is not to be blamed for the murder of Julius Caesar?Cassius is being put on trial (in the modern day) for the...

That's an interesting one. I'm not sure what you suggest
is actually what's going on the text; I'd argue that Brutus, Cassius and Caesar himself
all have a role to play in why Caesar gets murdered - it's not simply down to one person
or the other.

But, if I was in your shoes, and I had to
make the argument, I'd be trying to get the judge to find Brutus guilty of the
murder.

Firstly, Cassius knows that he alone cannot bring
enough conspirators on board. He needs Brutus - who is thought to be noble, and is one
of the most respected men in the city. Here's Cinna and Cassius discussing how they're
going to get the conspiracy on the road:

readability="12">

CINNA
O Cassius, if you
could
But win the noble Brutus to our
party--

CASSIUS
Be you content: good Cinna, take this
paper,
And look you lay it in the praetor's chair,
Where Brutus may
but find it...

So Cassius
wins Brutus over, yes. But who is it who makes the decisions later in the play? In
Brutus' orchard (at the sort of first meeting of the conspiracy) who decides that Antony
will not be killed? Cassius is arguing that Antony should
be killed along with Caesar. Brutus shouts him down. And Brutus says that
Antony shouldn't die. And that's what
happens.

So, from this scene, who is it who is
clearly in charge of the conspiracy - and the driving force behind it?
Brutus. Who is the last person to stab Caesar? (Significant? Yes!)
Brutus. Who is it who immediately takes control after the murder? Let's have a look at
who gives the instructions:

readability="16">

BRUTUS
Stoop, Romans,
stoop,
And let us bathe our hands in Caesar's blood
Up to the
elbows, and besmear our swords:
Then walk we forth, even to the
market-place,
And, waving our red weapons o'er our heads,
Let's all
cry 'Peace, freedom and
liberty!'

If I was making
your argument, I would focus for a second on the beginning of the play - and admit that
you could argue that it is Cassius that gets Brutus involved. (That said, though, Brutus
is already withdrawn at the start of the play - is he already thinking of murdering
Caesar? Are these his 'conceptions / only proper to
[himself]'?

I'd then move on to demonstrate that Brutus,
though is clearly the mastermind of what happens - and focus on even Caesar himself
saying 'Et tu Brute?', not 'Et tu Cassius?'. Cassius is there, but Brutus is in the
driving seat.

Hope it helps!