He doesn't take the time to be relieved over Newt's safety, quickly bending down to grab the hammer before tossing it over to his brother. "Here!"

Theseus figures that Newt's in a far better position to land a lethal blow. And while he knows that killing is always a sort of last resort for either of them, he just trusts that he'll be able to go through with such an act. It's why Newt called for the weapon, right? They took this job together and knew that this would have to be the end result.

...He'll talk to Newt later on, when this is all over.