The gemara tells about Antoninus who would take along with him 2 servants when he went to visit Rebbi, one he would kill when he reached Rebbi's house and the other when he returned home. Tosafos asks how he was able to do this since murder is a violation of one of the 7 Noachide mitzvos? Tosafos strengthens their question by saying that it is forbidden even for a Jew do be involved in the murder of a goy, so aside from the question on Antoninus himself, it is a question on Rebbi how he was able to condone this activity? Tosafos offers 2 answers: 1. Antoninus was afraid that the servants would report back that he went to visit Rebbi which would be threatening to his own life, therefore they had a din rodef and he was allowed to kill them. 2. He specifically chose servants who were mosrim and minim who are considered destructive and allowed to be killed.
There is a lot to analyze in each of Tosafos answers.
The first answer of Tosafos is somewhat difficult. First of all, Tosafos seems to understand that the concept of Rodef applies even to goyim. There is a discussion in the P'rashas D'rachim (derech ha'asarim, drush sheini) of the Mishneh L'melech regarding goyim and their obligation to sacrifice their lives for the 7 mitzvos. He the Mizrachi who says that goyim are obligated to sacrifice their lives to avoid violating any of the 7 mitzvos, but adamantly disagrees and holds that a goy is not allowed to sacrifice his life to fulfill any mitzvah (even one of the big 3). However, when it comes to murder the mishna l'melech holds that since it is derived from a sevara, even a goy is obligated to sacrifice his life and not kill. However, a goy is certainly not obligated to be more strict than a Jew. The Mishna l'melech will holds that whenever a Jew would be allowed to kill to give up their life, a goy can do the same.Tosafos seems to support the opinion of the Mishne L'melech and holds that Antoninus didn't have to sacrifice his life to avoid murder, rather he was allowed to kill his servants since their existence threatened his life. Another point on Tosafos is that Antoninus was allowed to intentionally put his servants in a situation where they would be endangering his life, thereby making it permissible for him to kill them. How can he do this? Perhaps the situation was that without the protection of the guards his life would be in danger, so he was allowed to take them to protect himself, and then kill them to protect himself.
Regarding the second answer of Tosafos - The Beis Yosef (y.d. 158) holds that the halacha of "ein moridin" means that one is not required to kill them but would still be allowed to kill them. The Taz disagrees and holds that it is assur to kill those who are listed as "ein moridin". The Taz proves this from our Tosafos who uses the concept of "ein moridin" to ask how Antoninus was able to kill his servants. Clearly Tosafos holds that "ein moridin" means that it is forbidden to kill them. The Nekudas HaKesef justifies the position of the Beis Yosef. It would be forbidden to kill goyim if they fulfill the 7 mitzvos, but if they violate the 7 mitzvos and technically deserve to be punished by beis din, it is permitted to kill them. The Nekudas HaKesef points out that our Tosafos seems to disagree because Tosafos could have simply answered that Antoninus chose servants who violates the 7 mitzvos, rather than saying he chose minim. The Nekudas HaKesef reconciles the Beis Yosef with Tosafos by saying that although it was technically permitted to kill any violator of the 7 mitzvos, Antoninus had no way of being certain that someone was a violator, therefore he chose those who did more severe aveiros out in the open such as minim. Furthermore, when Tosafos says that he chose "mosrim" and "minim", Tosafos meant to include all violators of the 7 Noachide mitzvos.