First don't get fixated on the "entire" humanity part. Even 10% should be unacceptable period.

Second you can't compare it to just one person's capability.

And it's never a doubt that really smart and insane people absolutely could kill a lot of humans considering modern technology. The reason they don't is because our society was historically built so that smart people don't want to or get stopped. For example ethics, religion, mental hospitals, self preservation instinct, police etc.