To the uninitiated, what Sirius was referring to was the hard and difficult times which prevailed during the First Wizarding War against Lord Voldemort. He was referring to Bartimus Crouch Sr. and his policies against those accused of being either Death Eaters or being involved with the Dark Lord in any way.
What he meant was that dark and difficult times bring out the best out of some people and the worst out of others. His general tone suggested that those dark times had brought out the worst out of Bartimus Crouch Sr. as he had become almost as ruthless as the Death Eaters themselves.
Examples of his ruthlessness: He authorized the use of unforgivable curses against the accused and threw a lot of accused (including Sirius himself) into Azkaban without a trial.
P.S. : Dear Potterheads, I know this might seem irritating to most of you as, for us, this quote means a lot. But when I read it from newcomer’s perspective, I realised that for someone who is not as familiar with Harry Potter quotes this will sound like really random words. Hence the explanation.