I imagine it's not the same to kill a nameless, faceless "enemy" and to kill a man with a face you can describe and remember, and after you realise that he has a name and some life he'd like to return to--just as you do.
Both sides usually think "the other one" is the evil and "we" are the good guys.
That said, the Cardassian doesn't think about the Jem'Hadar as "we", really... Did they kill any Cardassians? Who knows?
In a Cardassian library
or in a Cardassian gallery
"Reagan, it appears, is really only an ardent unionist if the unions in question are in Poland" - Stephen King, Skeleton Crew