Dear Tzungkuen,

>> t> etam. hi yajamaanassa, seyyo hoti na paapiyo
>>
>> etam.: this (acc/gen m. or f. of eso/etad/esaa)

Here 'eta.m' is genitive case.

>> t> yajamaanassa: (yajati) ppr.m.sg.gen/dat. 'performing a sacrifice'

Here 'yajamanassa' is genitive case.

>> t> seyyo: (seyya) meaning 'better'.
>> t> hoti: meaning 'is'
>> t> na: meaning 'not'
>> t> paapiyo: (paapiya) meaning 'worse'

'Seyyo' and 'papiyo' are adjectives.

>> Indeed (sacrifice) of such sacrifice-performer is better, not worse.

t> And why is the useage of Genitive ?

Genitive here denotes simply belonging, which it usually does.
The missing subject 'ya~n~no' is clear from the third line of the
stanza which you did not post - 'ya~n~no ca vipulo hoti', etc.

t> Can I find the example in "Introduction to Pali" by A.K.Ward?

I don't have this book.

With metta,
Dimitry