A backslider is the person who does that: someone who had improved, then started doing the old bad things again. The -er on the end is the ordinary English way of naming the person who does the verb. You hear it most in moral or religious talk — a preacher warning about backsliders — though people can use it more lightly about someone who quit sugar and then ate a whole cake. It names the person, not the act; the act is backsliding. It is a bit old-fashioned and a bit judging, so it can sting. Though actually, outside church and similar talk, many people would just say “he fell back into his old habits” and never use this noun at all.

故態復萌的人；走回頭路的人