。
Bad deeds always come back to you.
Literal
Dirty action [genitive-の] reward [topic-は] without-fail self [target-に] return-come.
An aphoristic statement about karma. 報い ('reward, recompense') is a noun referring to the consequences of one's actions, often negative (the English 'reward' is closer to ご褒美 in positive contexts). 自分に返ってくる ('comes back to oneself') uses ~てくる to mark the change-of-state coming toward the present/speaker. 必ず ('without fail, certainly') intensifies the certainty.