هتلر: جذور الشر
the word جذور carries the semantic connotations of the word "pathology", since pathology is all about the origin, development and buildup of diseases, also the word جذور implies both the fully grown شجرة as well as the stages (in other words, the development) that these جذور passed till its final growth into a fully-grown tree.

also,
هتلر: الشرارة التي أحرقت أوروبا
هتلر: الطاعون الذي اكتسح أوروبا
which carries the developmental and fatal aspect of diseases.(you will find it very attractive if you place it in red bold font and put the second phrase beneath the word "Hitler"

the following translations represent the core of pathology, but I don't find them attractive.

هتلر: دراسة في سيكولوجية الشر
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هتلر من الداء حتى الوباء
دراسة باثولوجية تسبر أغوار الشخصية التي كانت تجسيداً حياً للشر
(although rather long, it can be a possible title. However, the word "Hitler" has to be placed separately at the top and in a very bold font, while the rest should be put in a more thinner font underneath)
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هتلر: الداء الذي تحول إلي وباء اكتسح العالم
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هتلر: دراسة مرضية لبذرة الشر التي أحرقت ألمانيا والعالم
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هتلر
في أعماق الشر
the phrase إلي أعماق represents 90% of what pathology means.

Generally speaking, I don't prefer to translate book titles according to their literal meaning, I must do some "arabization" or rather "egyptianization" of both the title and subtitles in the book. You'll have hard time trying to translate

titles if you don't try to manipulate them into something more attrative to the Arab reader.
There can be many other possible translations, and it all depends on your creativity and sensitivity to the whole work.

Regards,
Mahmoud Helmi