Massacres.

[chambers]

effort isnt a factor when using the word massacre and neither is genocide.
massacre is not an attempt to wipe out a certain kind of people or amount. massacre simply means a lot.

i see wher eyou are coming from but they are only related by the fact that they are both about killing (often) a lot of people.

[Garlannd]

This should clear it up.

Main Entry: 1mas·sa·cre
Pronunciation: 'ma-si-k&r
Function: noun
Etymology: Middle French
1 : the act or an instance of killing a number of usually helpless or unresisting human beings under circumstances of atrocity or cruelty
2 : a cruel or wanton murder
3 : a wholesale slaughter of animals
4 : an act of complete destruction <the author's massacre of traditional federalist presuppositions -- R. G. McCloskey>
Taken from Websters.
*Edit*
Helpless seems to say little or no effort to me. Also my persnal opinion of when itachi wanted to know what he was capable of is that he wanted to know if he could be a horrible monster and kill all those he loved.