I believe the majority of cheaters "Justify" the act in their minds before cheating. "Regret" not "guilt" only comes into play if they get caught or the other person they cheated with causes them problems.
There are basically 3 types of cheaters.
1. The Incessant Cheater
(This person flirts all the time and believes variety is the spice of life. Most likely they have never had a monogamous relationship) - No guilt
2. The Unbelievable Opportunity Cheater
(This person had no plans to cheat but someone out of their league, very attractive, famous, or powerful made themself available for one night and they caved in due to temptation)
It's not unusual for this cheater to confess in order to "selfishly remove the burden off (their shoulders)".
Very often she/he feels guilty
3. The Justified Cheater
(This person feels unloved, unappreciated, taken for granted, or is not having their sexual or emotional needs met). In her/his mind they feel "justified" seek out other options to address their needs.) Rarely feel guilty because they blame their partner for forcing them to pursue other avenues. They may regret hurting the person if they get caught but they don't feel guilty for their actions.
My guess is most cheaters fall under #1 and #3 which translates into most cheaters feel no guilt.