Well, that is a fair question, and actually a fairly common one, but it is based on a misunderstanding. (These misunderstandings have been common ever since the Christians have mass produced their completely inaccurate and selfserving rendering of what they have renamed the “Bible”. It’s sad, and it has been responsible for more distortions of G-d’s truth than anything else that has ever existed.)
In the Book of Esther 8:11, the nature of the king’s second decree is described: “. . . to the effect that the king permitted the Jews of every single city to organize and DEFEND themselves; to destroy, slay and exterminate every armed force of any people or province that THREATEN them…”
And in 9:2, we find: “The Jews organized themselves in their cities throughout all the provinces of King Ahasueraus to attack THOSE WHO SOUGHT THEIR HURT…”
The original decree sentenced us all to death, with no recourse, no defense, no place to run and hide. The king refused to rescind that decree, but agreed to allow us to defend ourselves. We did not seek to kill anyone, nevertheless we enjoyed a victory. Killing is a bad thing, but being victorious over those who wanted to harm you is a victory and a joyous thing.
The fact is, we did not simply indiscriminately kill people, and not even all of Haman’s family was killed. As a matter of fact, our history records that the grandsons of Haman converted and studied Torah in the prestigious academy at Bnai Brak in Israel. (And you had to be pretty advanced to be allowed into that academy. That was the academy where the famous Rabbi Akiva studied and taught, centuries later.)
The enemies wanted to kill the Jews, but the Jews killed them. Therein lies a significant difference. They wanted to kill us, but we killed them only out of necessity (self-defense).