Those who were able traveled in larger parties. It was probably much like the cruel teasing of inmates that goes on in prisons today. It represents a general jab at authority of any kind. A pulling of everyone down to one's own level. You're dying just like us.
Death is the great equalizer.
We are punished justly, for we are getting what our deeds deserve. These taunts, however, are making the other condemned brigand very uncomfortable. To stand by and participate in such an unrighteous act as to execute an innocent man is an impious, sinful act, and the second brigand refuses to desert his sense of right and wrong. Jesus' disciples have fled or linger disillusioned at the margins of the crowd. But here on the cross to one side, a fellow condemned man, life ebbing out of him, looks across and sees not another dying man, but the Messiah himself.