Coincidences sometimes happen. Since those other dogs also died very close, the snake could easily have gone to their property too – why not? Envenomation from a Siam Russell Viper is extremely powerful and deadly – it is likely to have enough venom to kill several dogs. Adult snakes don’t have to release all their venom at one bite – they can control the amount of venom delivered, or even choose to bite without envenoming. Juveniles can actually be more dangerous since they haven’t learned this skill yet. I’ve never seen bleeding in the rats I’ve poisoned (proprietary rat poison) and I’ve never seen bleeding in dogs poisoned with insecticide. Note also that dogs are often drawn by the presence of a snake, and if one interacts with it, the other is bound to join in.