He was assassinated by a number of Senators, among them the infamous former friend, Marcus Junius Brutus, hence the famous cry "Et tu, Bruté?".
I think that was Ruben's point - Senators who pretended to be Caesar's friend, but secretly plotted against him. Modern parallels seen in Senators who might appear to laud our new US president, but who look in secret for his first mistake, his first error, which might give them cause to cry for his impeachment - it is ever thus, and history repeats itself.
As to declaring himself king, I don't know if Ruben intended any such comparison or not.