It's mostly just an observation, and it may not actually be the case. His ability of "Life-Minus" does not simply transfer damage onto one of the hosts of his straw dolls. Instead, he swaps his physical state with them.
- When Hawkins used his Straw-Sword, Law used the momentum of his missed swing to use Amputate on the surrounding subordinates who were under Hawkins' ability. Hawkins was shocked, and was forced to block Law's next attack.
There's two main reasons why I think this is the case. 1) When Hawkins fights, he never allows those under his influence to go into combat themselves and 2) Every single person he has used his power on is in perfect condition.
Basically, there's the off-chance that Hawkins would be split in half if he used Life-Minus on one of the affected fodder. His weakness is simply: "Get rid of his extra lives before attacking him directly". I'm still waiting for it to be true, and if Killer makes note during their fight, I would be quite satisfied.