Why does Caesar say he would fear a man like Cassius

Cassius was a man of principle who believed in the Roman Republic.

As the Republic had failed due to aristocrats attempting to rule it to their own benefit (and not republican principles: res publica = things for the common good) Caesar made himself dictator for life to control these self-seekers, but knew that they could fix this by taking his life.

So he feared this threat from the self-centred aristocracy. Brutus was a man of principle, but the others were quite comfortable in using him to gain their own ends.