The word كُلُّ means “every,” so the statement is general rather than restricted to one group. In كُلُّ عَبْدٍ, the first word is the muḍāf and the second the muḍāf ilayh. The latter therefore carries a genitive ending and tanwīn.
A person can become careless through confidence in their current conduct or hopeless because of past mistakes. The report points instead to consistent reform.
As a student cannot rely on one successful exam for an entire course, a person cannot treat one good beginning as a substitute for persevering in good conduct.