Commentary for Chullin 2:8

השוחט לשם הרים וכו' שחיטתו פסולה – an idolatrous offering does not prohibit one from deriving benefit from it, since all of these [things] are not done for idolatrous [purposes], as it is written (Deuteronomy chapter 12, verse 2): “[You must destroy all the sites at which the nations you are to dispossess worshiped] their gods, whether on lofty mountains [and on hills or under any luxuriant tree],” and not on the mountains of their gods. But however, it is prohibited to eat it, because it is similar to slaughter for the sake of idolatrous [purposes] by exchanging something [of their own in its place]. And specifically if he said: לשם הרים, לשם גבעות–in honor of mountains, in honor of hills (which is the language used in the Mishnah), but if he [the person making the offering] said: to the angel appointed on the mountains and on the hills, this is considered the sacrifices of the dead and it is prohibited to derive benefit [from them].

Introduction Our mishnah deals with a person who slaughters an animal for idolatrous purposes.

If one slaughtered [an animal] as a sacrifice to mountains, hills, seas, rivers, or deserts, the slaughtering is invalid. If a person slaughtered an animal as a sacrifice to a natural phenomenon, such as a mountain, body of water or desert, the animal is invalid, even though it was slaughtered properly. Note that worship of mountains and hills was also referred to in Mishnah Avodah Zarah 3:5 and seems to be based on Deuteronomy 12:2.

If two persons held a knife and slaughtered [an animal], one intending it as a sacrifice to one of these things and the other for a legitimate purpose, the slaughtering is invalid. We learned in mishnah two of this chapter that if two people jointly slaughter, the slaughtering is valid. Both are participating in the slaughtering. Therefore, if one of them intends to slaughter the animal as an idolatrous sacrifice, the animal is invalid.