You will come across the Demons fairly soon in your travels and they aren't too hard to kill. I was worried about absolving and punishing them and whether it would count or not. I had been doing this nearly 98% of the time (large Holy bonus) and I was unsure if it was counting towards the 30 "kills". Since my scare I started working at just killing them and at about three or four kills I got the achievement. That provided me with enough proof that you can in fact absolve and punish for the achievement.