The insect in your photo preying on the monarch caterpillar is the immature stage or nymph of the spined soldier bug, scientific name: Podisus maculiventrus, in the family Pentatomidae. Bugs in the family Pentatomidae are often referred to as stink bugs, because they produce potent odors to ward of predators. The majority of them are plant feeders, sucking juices from the stems, leaves and fruits with their beak-like mouthparts. However, a few of them are predators, including the spined soldier bug. These species attack insects and other arthropods and suck the body fluids out for their meals. Stink bugs do not have their front legs modified to grasp prey, so these predatory bugs have developed a very toxic saliva that quickly kills their prey. The bugs usually sneak up on their slow-moving prey and quickly stab it with their beaks, injecting their paralyzing saliva. And as you have seen they can attack prey items much larger than themselves. And the sequestered poisons inside the monarch caterpillar have no effects on the stink bugs. These bugs can be quite common, and they are considered beneficial because they fed on many kinds of pest insects. They only incidentally happen to come onto milkweed, so they probably have little overall impact on monarch caterpillar populations. I have included a link below to a webpage with more information and photos about these stink bugs.

 http://entnemdept.ufl.edu/creatures/beneficial/podisus_maculiventris.htm