I just started running an affinity deck, and I have a question about arcbound ravager. If my oponent plays diabolic edict, and I have arcbound ravager and memnite in play, can I choose the memnite to sacrifice and then activate arcbound ravager's ability on memnite? Thanks in advance!
You can activate Arcbound Ravager's first ability in response to Diabolic Edict as you can in response to almost any other spell (C.R. 117.1a). Like any other spell, Diabolic Edict has no effect unless and until it resolves (C.R. 609.1, 608.2c; see also this thread and this thread). However, note that activating that ability this way will not counter Diabolic Edict, so that when that spell resolves, the targeted player still has to sacrifice a creature if possible.
can I choose the memnite to sacrifice and then activate arcbound ravager's ability on memnite?
It's legal play to do that, but it will not achieve what you hope for. You'll still have to sacrifice some (other) creature to Edict.
One sacrificed creature cannot satisfy both Edict and Ravager,
One sacrificed creature cannot satisfy both Edict and Ravager,
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules