“she.” The definite article is feminine and agrees with the pronoun “her” at the end of the phrase.
“keeps on killing.” The Greek word apokteinō (#615 ἀποκτείνω), kill, is a present participle. She kills and keeps on killing. The translation, “is killing” would be appropriate in some contexts, but not here. The point is that she has killed the prophets and keeps on killing them, something that was about to be fulfilled in Jesus himself in just a few days.