For decades, prisons have been described as "schools for crime" or "breeding grounds for crime." The central idea is that incarcerating offenders makes them worse. This is because they can learn new "tricks of the trade." There is an inevitability to this perception, i.e., that if you land in prison, you will be a far more corrupt or violent person when you leave.
It is well-known that among the main topics of conversations in prison are crime and drugs. Inmates boast about what they have done. They talk about plans for the future. Some commit crimes while incarcerated and plan new crimes that they will commit once they are released. Some engineer crimes while confined that others will commit upon their orders outside the institution (e.g. "shot callers" or gang leaders whose influence reaches beyond prison walls).
I have interviewed numerous offenders in different institutions about whether prison makes a person worse. Their answers may be surprising to some people. Inmates emphasize that choices are continually made about how time is served. Some aspire to be prison kingpins. Confinement is just one more arena in which to conduct criminal operations. Others, however, make a different decision. They adhere to the prison rules and policies, remaining out of the "action." Fed up with the lives they have lived, they desire to change or, at the very least, not to risk getting new charges which will extend their time in prison. They participate in programs, try to get along with other inmates and with institutional staff.
The inmates who stay away from prison criminal activities remain cordial to other inmates. Not wanting to be thought of as "snitches" or informants, they remain affable and participate in a variety of activities (that do not violate the rules. But they draw the line at committing new crimes. Some develop disgust (that they keep to themselves) with their fellow inmates who are perpetually coming up with new schemes, con games, and manipulative maneuvers. They report that others leave them alone and do not try to pressure them into involvement in more violations and crimes. They have found that, often, other inmates respect them.
In short, just as he did in the free world, an inmate chooses the people with whom he develops close associations. He makes decisions about the type of person he wants to be. He decides what temptations he will resist. By no means is it inevitable that he will become a more hardened criminal or a more dangerous person because he is serving a sentence in a correctional institution.