What was Casca's motive for killing Caesar in the play Julius Caesar? When in the play does he show this?

Casca is worried that the senate will name Caesar king and enslave all of Rome.

Cassius describes Casca as “sour.”  He is definitely more hardened than the rest of the group of senators who are involved in the conspiracy to kill Julius Caesar.  He provides a cynical description of Mark Antony’s offering Caesar a crown and the people’s reaction to it.

Casca also describes the punishment of Marullus and Flavius, the two we saw in the first scene who complained to the crowd about celebrating for Caesar when they used to love Pompey. 

I could tell you more
news too: Marullus and Flavius, for pulling scarfs
off Caesar's images, are put to silence. Fare you
well. There was more foolery yet, if I could
remember it. (Act 1, Scene 2) 

From Casca's perspective, the "silence" of Marullus and Flavius is unfair, and a warning to them all to be careful about speaking against Caesar.

Casca also complains about bad omens that he sees on the way to Brutus’s house.  He believes they are “they are portentous things” (Act 1, Scene 3).  Casca is worried that the senate will offer Caesar a real crown, and he and every other Roman will become a “bondman.” 

Indeed, they say the senators tomorrow
Mean to establish Caesar as a king;
And he shall wear his crown by sea and land,
In every place, save here in Italy. (Act 1, Scene 3)

It seems that Casca was the tough guy of the group.  He is appointed to stab Caesar first, and in fact his wound was the fatal one.  Everyone else was stabbing Caesar as he was bleeding out, just trying to get a jab in.  Casca did not necessarily have a personal cause against Caesar, but he did not want him to become king.  He took part in killing Caesar for political reasons.