The question in the OP is pretty vague. What kind of "ruthlessness, cruelty, or evil" qualifies? Is the OP talking about sadists who deliberately stomp on kittens' heads and crush them just for laughs? Or is the OP talking about the normal competitiveness, lack of empathy, and petty cruelty that all of the different personality types routinely indulge in?
Example:
If you get a little power (like when you hold a government or bureaucratic position) then you have to follow policy. But there are always trade-offs to policy: It helps some people and hurts others. So you implement policy and some people end up getting hurt, and you shrug your shoulders over it: It wasn't you, it was the policy. The ones who ended up getting hurt shouldn't have fought the policy.
Lots of NFs end up in positions of power (at least in non-technical fields and careers) due to good people skills. Personally I've seen a lot of INFJs in leadership roles, being an even-keeled type but also a Feeler with good people skills. But I've also seen all the other kinds of NFs acting as leaders and bureaucrats as well, including myself. Anyway, exercising power includes getting used to the idea that your power is going to hurt some people, and apparently NFs can accept that.
Also, NFs can be kind of passive-aggressive (Example: Rationalizing questionable actions by insisting that their intentions are good). Passive-aggressiveness can become magnified by power, leading to ruined careers and lives. In that respect, the bluntness or directness of other personality types would be preferable; at least you know where you stand with them.