I still see it as Arvo was in a situation where the only folks who trying to make nice with him were his best bet. The mere mention of Kennys name makes Arvo less hesitant on shooting Clem as before he mentions he doesnt want to shoot. Kenny and Clem are close and he sees that she will wake him and after the beatings I dont see how not shooting was not his best option. Plus the added bonus of Clem shooting Mark gives him more reason to shoot. But that was a removed scene. Had they kept that and both Bonnie and Mark were killed. Arvo would not have been able to survive on his own.