In Old School Runescape killing abyssal demons goes a lot slower than in Runescape 3. In OSRS you will kill about 60 of them an hour if you are using melee, if you are using magic, you can likely get up to 200 kills or more. To get to this number, you will have to pay a lot of attention and use ice barrage.  In Runescape 3 you can kill up to 1200 abyssal demons in an hour with maxed-out stats and gear.