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:
O Cassius, if you could
But win the noble Brutus to our party--
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:
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 (That said, though, Brutus is already withdrawn at the start of the play - is he already thinking of murdering Caesar?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!
You could also argue that it was not murder, but rather an act of war in opposition to a tyrant and in favour of the good of Rome. This, in effect, is what Brutus attempts to argue to the Roman people over Caesar's dead body.
The Romans, at the time of Caesar's assassination, took very seriously the idea that they would rule and govern as body of men -- The Senate. It could be argued that Caesar's actions in appearing to assume the lion's share of power as king were treasonous, for which he should be executed.
The idea that there are political acts and acts of war that supersede the "normal" course of law and order is very common within governments. Just as today, the US holds political prisoners that are jailed and, in some cases, executed without any sort of due process.
The motto of this line of defense in favour of Cassius' innocence -- Brutus' words at Caesar's funeral: "Not that I loved Caesar less, but that I loved Rome more."