Thing is, see, Xtians can tell atheists they’re pieces of shit because the Xtian dogma presupposes that everyone is a piece of shit. But if - and only if - you get god, can your being a piece of shit be okay. Without god you’re just a piece of shit. Nothing personal, it’s just that, you know, god says.
"Everyone is a sinner" is a trope used by the powerful to control the majority since forever, and its use trancends religion.
As the leader (chief, president, supreme ruler, first secretary, high poobah, or whatever), it is hugely valuable to be able to arrest (execute, exile, beat, or imprision) literally anybody that you fear might be a threat to your power and authority. But it's also dangerous to be seen to be disregarding the rules - your entire schtick is that everyone should obey (you), so rules are paramount.
The obvious solution is a set of rules (laws, commandments, regulations, traditions) that is sufficiently complex and even internally contradictory that everybody is guilty of
something.
Hey, presto! Now we have a society in which arbitrary and whimsical punishments are not only permitted by the rules, but are accepted by the people as normal, reasonable and just!
This is helpful if you are Joe Stalin, and want a veneer of respectability on your actions in sending dangerous intellectuals to the gulag; But it's even
more helpful if you are the Pope, and want an excuse not only for excommunicating anyone who threatens your power, but also for an allegedly moral and loving God allowing a person who has led a fairly blameless life to suffer some dreadful calamity.
Your house was flattened by a tornado (tsunami, earthquake, stampede of bison, invading barbarian horde)? Why didn't God protect you? Because you are a miserable sinner!
As long as the rules are carefully crafted to ensure that it is basically impossible for any human not to break at least some of them (lustful thoughts, anyone?) you can persuade
everybody that they deserve to be punished, and you can get away with unjustly punishing
anybody at all - many of them won't even complain, as they are wracked with guilt and genuinely believe that they deserve whatever you mete out.