With the kind of power flowing through Seth, he believed there was a chance that if he meant it and went all out, he could perhaps have his hand driving through Rose's chest, particularly her heart, and killing her, but he didn't take this chance. Not to mention that it was just a chance and not an assurance, there was also the fact that Seth wasn't out to kill Rose.
The fact that she wasn't also hard to kill made it a lot easier to eradicate thoughts of killing her, besides, Seth had other plans with his palm strike. As much as Rose's plan was to subdue him, Seth's plan was to restrain her, so as his palm hit Rose, he quickly pushed his qi into her, his plan being to neutralise ir out her out of commission by poisoning her with his qi.
