I giggle to think of the circumstance under which they had to make the 3rd rule.

Hey, just a suggestion. The rhythm and structure of the three commandments would flow just a bit better if you changed it to the following:
-Thou shalt not kill thy beast for greed.
-Thou shalt not force an evil deed.
-Thou shalt not lust upon thy steed.

My reasons for the changes are as follows:
1 - Using 'thy' with 'thou' is more consistant, using the 2nd person singular informal for both the possessive and nominative roles.
2 - Changing 'any' to 'an' in the second makes the syllables flow better.
3 - Upon fits the resulting meter from 1 and 2 better if it replaces the word after.