Power and need for control are strong motivating forces for evil. Which character in The Mockingjay displays this most predominately?
Power or a sense of it is something that can be misused and/or misinterpreted.
There are many characters in Mockingjay by Suzanne Collins who are have a need for control and power.
The character that predominantly fits this role in all three of the books is President Snow. In Mockingjay, Snow uses Peeta to control Katniss as well as the Rebels. Snow is a character who needs power and uses it to control the districts. He abuses that power by oppressing the outside districts, supporting the Hunger Games and lavishing the Capitol with food and luxuries to keep them appeased too.
You could look at Gale as a character who misuses power or plots to misuse power by developing weapons that will kill innocent people. This character study would be best done if you look most closely at the part where he is developing weapons with Beetee as well as when they are taking over the Nut in District Two.
Coin would be an interesting character study and look at why Katniss refuses to follow either Coin or Snow in the end of the novel. What is her purpose in using Katniss for the propos throughout the novel? Why does Coin send Peeta into the war with Katniss?
I think Katniss mostly works against wanting and needing power because she recognizes that it is an evil motivating source as you call it. She is not someone motivated by evil though.
although modt people might think the answer to this might be president snow, I believe the answer to this is president coin or even gale.
Most likely that character would be Katniss.