My theory is that breaking the steel heart makes them lose their invulnerability and they become a normal human. I doubt it kills them unless Oda wants the protagonists to start taking lives all of the sudden.
Nope destroying the heart kills them and you need to use CoC to do it, simply destroying it without CoC is just going to have the heart regen.
Also in the chapter Harald tells Loki to hit him right where the heart is, which of course after Loki did that, Harald died.
And yeah Oda usually doesn't allow Zoro and Sanji to kill their opponents, but this time it might be different.
Unlike with their past opponents, Sommers and Killingham are pure evil demons through and through, Oda has shown us that these two are not redeemable in anyway, they have done the most horrible and heinous stuff, a simple defeat won't do it, they need to die.
Finally throughout the chapters on Sommers and Killingham, Oda has been hinting on them possibly being killed.
Zoro questioning about Sommers' heart which of course shocks Sommers, Zoro telling Sommers that Sanji is going to kill Killingham, Killingham telling Sanji to try to kill him, and finally again in this chapter where Zoro is thinking about how kill Sommers.
There is also the fact that after Harald died, Imu got a bit more weaker, that was probably foreshadowing the possibility of Sommers and Killingham being killed on Elbaf, which would not only weaken Imu but also force him to retreat.
