The following is not a direct instance of the practice you're asking about, but I think it sheds some light on some Christian conversion practices during the early Middle Ages. During Charlemagne's conquest and forced conversion of the Saxons, intimidation and manipulation were regularly deployed where gentler missionary practices had failed. Charlemagne's "Capitulatio de partibus saxoniae" criminalized paganism and made refusing to be baptized punishable by death. The Paderborn Epic also illustrates the Frankish attitude well: "what the contrary mind and perverse soul refuse to do with persuasion, let them leap to accomplish when compelled by fear." We find not infrequent stories of mass baptisms by the sword and by trickery (blessing the water upstream while a group crosses at a ford downstream) during the conversion of the Germanic tribes in the 5th through 9th centuries, but it's easy to overlook the political significance of conversion.

Arian and Catholic Christianity were in ideological and often direct military conflict, and each new tribe brought in to one or the other sect could mean a significant advantage down the road. It could be argued that if Clovis hadn't (voluntarily) converted to Catholicism and instead adopted Arianism like most of the other Germanic tribes, the ideological dominance of the Catholic Church might not have been secured by Charlemagne centuries later. That story actually returns us to our original question, as we see Clovis kill his pagan Frankish rivals when they dispute his adoption of this foreign roman religion.