。
Hitler must have been a thoroughly wicked man.
Literal
Hitler [topic-は] afterlife [gen-の] bad person [probably-だろう].
後生 here is the Buddhist noun for 'the afterlife / one's next life.' The set phrase 後生の悪い literally means '(someone) whose next life will be bad' — idiomatic for 'morally reprehensible / wicked,' invoking Buddhist ideas of karmic consequence: someone who has accumulated so much bad karma that their rebirth will suffer for it. だろう supplies the speaker's 'probably / must be' hedge, turning the characterization into a confident inference.