By the time one encounters them in the work of important philosophers, such as Kumārila and Jayanta (in the Sarvāgamaprāmāṇya section of his Nyāyamañjarī, henceforth SĀP), the Saṃsāramocakas were probably only a straw-man kind of opponent, one which everyone agrees to condemn. In other words, nothing in the SĀP leads one to infer the existence of a specific group of heretics bearing this name in Jayanta's time and place. Rather, in the SĀP (as in other philosophical texts, see again Halbfass' essay mentioned above) they cover the role of an ideal opponent, since their existence enables one to question a purely rational foundation of dharma. As shown in Kumārila's dealing with them (ŚV codanā 201ff, and especially 243), since the Saṃsāramocakas kill other creatures for these creatures' sake, ''not harming others'' or ''helping others'' are not sufficient criteria for recognising a valid religious belief. After all, the Saṃsāramocakas kill locusts, etc., (and would kill human beings, if possible) in order to save them from the bondage of saṃsāra. From a subjective point of view, they are benefitting their victims and, unless one accepts an objective standpoint to judge about morality, there would be no reason to blame them.
Can one establish the validity of a religious behaviour through arguments which avoid any appeal to an authority?
On Jayanta see this post. On his Sarvāgamaprāmāṇya section, see this one.