Woulda' been nice if he kicked the bucket right then, but Szilard was immortal. Unlike Ennis, he wasn't made in the elixir, he just drank it. But he could still recover from that just as fast, and when he did...

[He sighs and stares at the tabletop. This is one of the parts of this story that he doesn't like so much, though the ending makes up for it.]

The thing about a homonculus is that there's a link between them and the person who made 'em. For Ennis, that person was Szilard. So right then and there, when he decided he wanted her to die, she started to, well... Break down, basically, in her cells. He didn't want to kill her quickly.