Sentry has no actual proof to back the supposed claim that he is some actual supernatural entity or deity of Death instead of it just being a relative term applied to him because of his recognized in-universe capability of bringing death on a massive scale.
So while he may or may not break down Lobo to a molecular level, he can't kill him. Higher powers than Bob or the Void deem otherwise.
It'd be like calling Jean Grey a "Death God" before the Phoenix Force retcon. Sure, she's immensely powerful and eating stars and all that jazz, but she's not an actual Death God or Angel or anything. Just an absurdly powerful mortal.
I agree he ain't no Death God. That's just people taking things literally.
But he has proven to be able to brake down particles with next to no limit due to his power resource, which once again people take literally as being at the "strength of a million exploding suns".
Regardless his powers are schizophrenic manipulation with near no limits so he will beat the brawler Looney Toon.
How though? Even if Lobo's body is destroyed in its entirety, Lobo has shown himself to be able to exist as a Disembodied Spirit and just body jumping until his body pulls itself back together.
It's both one of their powers along with Thanos. And you're right. I'm jumping the gun and I'm drunk.
I'm braking down the fight due to the fact that Lobo is an incredible powerful tank brawler looney tune. And Sentry is a Superman like schizo particle manip fighter who can defaet his opponents from a distance if need be.
This alone gives him the advantage to keep the killing happening to have an upperhand as far as I'm concerned.
It's a shame we don't have a locale for I'm imaging this happening in space.