Hmm
Akainu is one notch above for one reason, his bloodlust to slaughter the enemies and not leave ANY of them.
like literally, Kaido spared his enemy so many times so far:
1- Defeated Luffy in Kuri, he spared his life
2- supposedly killed Orochi, he OFFERED Orochi’s men to join him instead of dying
3- Defeated Scabbards on rooftop, still didn’t kill any of them and waited for Luffy and co to come up...
4- Defeated Luffy now, still didn’t cut his throat as he stated
5- Still offered the SHs and others from alliance to join him..
In most if not all of those cases, Akainu would have already gone for the killing blow... and that’s what makes Akainu scarier for me...