Pronunciation of "xeno-"

I actually want to present this question in two parts:

1) First of all, xenos comes from ξένος. Nevertheless, it is pronounced with "z" instead of "ks" (/ks/). How come?

2) Let's take the word "xenocide" (the killing of xenos - strangers). Should 'Xe' be pronounced as "ze" or "zee"? In popular culture, you usually see "zee" being used. But, in the context of xenocide, there is a clear connection with genocide (/ˈʤɛnəsaid/, [jen-uh-sahyd]). What should be preferred?

I always pronounced it "zee-no-cide".

For easy reference:

edit:

As another example, take "Xena".

Answer

English (and most other European languages) imported many of the Greek roots it uses a long time ago -- long enough that ordinary sound changes took place subsequently which modified the original Greek pronunciations. Just as we no longer pronounce Anglo-Saxon words the same as the Anglo-Saxons did, we no longer pronounce Ancient Greek words the same as the Ancient Greeks did. In particular the Great Vowel Shift of around 1500 CE significantly changed the way we pronounce vowels. Looking at the Greek eta έ [ɛː] vowel:

Middle English [ɛː] --> [eː] --> modern English [iː]

..giving us "ee" [iː] instead of "eh" [ɛ:] for the sound of the vowel in xenocide.

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The other part of your question is covered in this Eng.SE question. Basically, many Greek roots came to English through French and then were simplified to meet English phonotactic constraints. So Gk [ks] --> Fr [gz] --> En [z].

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(Interestingly enough, a similar thing happened in Greek itself, though not to ε. E.g. the letter η (eta) is now pronounced [ita], among many other changes.)