I know that in order to successfully cast AK, you need to mean it and have a powerful mindset at that moment.

However during DH, in the RoR Crabbe is using it twice in a really short period toward Hermione.

My question is, why doesn't Voldemort/Bellatrix/other powerful Death Eaters spam AK to kill.

One could argue that Bellatrix likes to "play with her food before eat it" and Voldemort wants to prove that he's above everyone else, so he probably wants to give a showdown of his talent before killing the target. But if he was afraid of Dumbledore, why did he need to duel him in a spectacular way in OotP instead of spamming killing curses everywhere ? And every other Death Eater should spam it too...