Diodorus Siculus, The Library of History 37.22a (Full text available on LacusCurtius)
“Sertorius, when he noticed that the uprising among the indigenous people was overwhelming, turned nasty to his allies: he accused some and had them killed; others he threw into prison; but he liquidated the wealth of the richest men. Even though he acquired a great deal of gold and silver this way, he did not put any of it into the public treasury for the war effort, instead he hoarded it for himself. He didn’t use it to pay the soldiers either or even share some of it with his commanders.
When it came to capital cases, he did not consult the council or advisers, but had hearings in private and gave judgments after serving as the solitary judge. He did not consider his commanders worthy of invitations to his banquets and demonstrated no beneficence to his friends. As he was generally driven mad by the worsening state of his own rule, he acted tyrannically toward everyone: he was hated by the people and conspired against by his friends.”
Ὅτι ὁ Σερτώριος θεωρῶν ἀκατάσχετον οὖσαν τὴν ὁρμὴν τῶν ἐγχωρίων πικρῶς προσεφέρετο τοῖς συμμάχοις, καὶ τοὺς μὲν καταιτιώμενος ἀπέκτεινεν, τοὺς δὲ εἰς φυλακὴν παρεδίδου, τῶν δὲ εὐπορωτάτων ἐδήμευε τὰς οὐσίας. πολὺν δὲ ἄργυρον καὶ χρυσὸν ἀθροίσας οὐκ εἰς τὸ κοινὸν τοῦ πολέμου ταμιεῖον κατετίθετο, ἀλλ᾿ ἰδίᾳ ἐθησαύριζεν· οὔτε τοῖς στρατιώταις ἐχορήγει τὰς μισθοφορίας, οὔτε τοῖς ἡγεμόσι μετεδίδου τούτων, οὔτε τὰς κεφαλικὰς κρίσεις μετὰ συνεδρίου καὶ συμβούλων ἐποιεῖτο, διακούων δὲ ἰδίᾳ καὶ μόνον κριτὴν ἑαυτὸν ἀποδείξας ἐποιεῖτο τὰς ἀποφάσεις· εἴς τε τὰ σύνδειπνα τοὺς ἡγεμόνας οὐκ ἠξίου παραλαμβάνειν, οὐδὲ φιλανθρωπίας οὐδεμιᾶς μετεδίδου τοῖς φίλοις. καθόλου δὲ διὰ τὴν ἐπὶ τὸ χεῖρον ἐπίδοσιν τῆς περὶ αὐτὸν ἐξουσίας ἀποθηριωθεὶς τυραννικῶς ἅπασιν προσεφέρετο. καὶ ἐμισήθη μὲν ὑπὸ τοῦ πλήθους, ἐπεβουλεύθη δὲ ὑπὸ τῶν φίλων.