Who can I safely kill with the Ebony Blade?

Solution 1:

You can use the Dead Thrall spell to repeatedly kill just one NPC who trusts you, instead of having to kill many different ones.

Solution 2:

Befriend anyone (such as http://www.uesp.net/wiki/Skyrim:Beitild) who is a Dark Brotherhood assignment. Use Dead Thrall and kill them again and again and again. That way, you complete the Dark Brotherhood Quest and complete the Ebony Blade

See here:

http://www.uesp.net/wiki/Skyrim:Ebony_Blade_Victims

Solution 3:

I keep resurrecting Cicero. He is obnoxious and a pleasure to kill. It charges the ebony blade nicely.