What are you a high school kid learning Lomdus? Lomdus has nothing to do with Halacha.
1. I don’t think that the Din of Goel Hadom applies during the time when there is no Arey Miklot.
2. Even if it does, since we are in Golus, you can’t put yourself in danger because if you kill s/o they will prosecute you. This is true even in Israel because they have secular rules. They don’t apply Torah rules esp. in a case of killing (whether permitted by the Torah or not)!