In Shakespeare's Julius Caesar, why does Antony describe Casca as envious in his funeral oration?

In Act 3, Scene 2, when Antony is addressing the mob he shows them the torn and bloody cloak covering Caesar's body in order to stir them up even further than he already has done. Antony, of course, has no idea which of the many holes and tears in the cloak was made by which conspirator, but he pretends to know who was responsible for each of them. At one place he says:

Look, in this place ran Cassius' dagger through:
See what a rent the envious Casca made:
Through this the well-beloved Brutus stabb'd;

No doubt Shakespeare needed an adjective to make an iambic pentameter line out of the reference to Casca and he chose the word "envious" more or less at random. Casca was probably no more or less envious than the other assassins, but by calling Casca envious Antony can suggest the idea of envy as applying to all of them. Evidently Antony is pointing to an especially large tear in the fabric when he mentions Casca. The "rent" could be made to symbolize an unusually vicious intention based on some especially reprehensible motive such as envy.

Antony was showing the mob Caesar's torn and bloody cloak rather than the body itself. There was probably no body under the cloak, because it would have been awkward for Antony to carry it in and difficult for the audience to see, since it was in a coffin and surrounded by the members of the mob. When the stage directions state:

Enter ANTONY and others, with CAESAR's body

they are probably carrying a dummy covered with a bloody cloak. They then place the dummy in the coffin still concealed by the cloak.

What Shakespeare actually did was to have two identical cloaks as regular properties for the performances of this play--except that one was in good condition and the other was all shredded and bloodstained.

In Act 2, Scene 2, Caesar decides to go to the Senate house in spite of his wife's warnings. He says:

How foolish do your fears seem now, Calpurnia!
I am ashamed I did yield to them.
Give me my robe, for I will go.

The scene does not end there. Caesar has considerable additional conversation with his visitors while the good robe is brought to him and the audience can see him putting it on. Then when Antony holds up the other robe (or cloak or mantle) to supposedly reveal Caesar's body, it appears to be the same robe the audience saw in Act 2, Scene 2, but now all torn and covered with dirt and blood. The audience never sees the body because it isn't there--but the mob supposedly gazes at the body in the coffin and reacts accordingly to the sight of the mutilated Julius Caesar.

The bloody robe is effective because Antony can hold it up for everyone in the theater to see, whereas the body, if Shakespeare had tried to show an actual body, would have been horizontal, hidden inside a coffin, and concealed from view by all the members of the mob.