In Machir Yayin, the Rema writes that the verse’s use of the phrase “al nafsham” (“on their souls”) rather than “al gufam” (“on their bodies”) implies that the Jews’ targets were their spiritual enemies, not their physical enemies. In other words, Jewish survival depends upon their defending themselves from sin.
The Sfas Emes focuses on the word al (“on”). He explains in this context that teshuva out of a sense of love is greater than teshuva out of fear. According to him, the Jews were on a higher level at this point – no longer threatened with annihilation – and the verse therefore uses the word al.