Indeed, combining two very different things that may be viewed as "good" on their own does not necessarily produce another "good".
Most schools of Daoism and Buddhism (in particular the earlier, less diluted ones) have little to nothing in common with Zen - the Japanese pronunciation and reinterpretation of the Chinese Chán - the former having varying degrees of explicit or implicit morality, whereas the Japanese Zen (and to a lesser extent, its preceding schools of Chinese Chán) is first and foremost an amoral tool - and like all tools, can be used in many different ways for many different purposes.
Can Chán/Zen be applied to warrior arts and to preexisting societal models, as it was in Japan? It most certainly can. Was that its original, historical purpose? No, it most certainly was not. However, as with any tool, it is inherently amoral and can be applied however the wielder wishes it to be, with varying degrees of success and consequences. A knife can be used to chop a carrot in preparation for a meal or carve a stick into a flute, or it can be used to slit a man's throat while he sleeps. The knife itself is amoral.
Zen - and to a lesser degree, Chán - is a tool. Nothing more, nothing less. When applied to the preexisting, highly authoritarian, societal models of Japan, it can very easily - and as we know, historically did - produce what was effectively a death cult. Or, to be more precise, it could - and did - produce a society of self-identified living "tools", whose entire purpose of existence was to serve their lords and masters to the best of their abilities, even (or perhaps, especially) unto death.
And when those lords and masters are highly aggressive, xenophobic imperialists, well...
