It's quite interesting how Rhino's and Hippo's behave. I'm not sure what's behind them assisting other animals, I do know something like a Hippo, although it may initially assist another animal, if it creates too much disturbance, it can also suddently turn and kill it. My assumption is the big creatures don't like their peace being disturbed, so they try to remove the disturbance alive, if that fails their dead! Although I've no idea the real reason behind this behaviour.
Dunno about your Hippo theory. They are the biggest human killers in Africa (or so i was told). They are very territorial and extremely dangerous to humans who think they aren't. Not sure what they are like with other mammals.
Don't know about humans. I have seen them rescue other animals, but as I said if it don't work out, they are likely to kill them. Probably for the reason you have given, that they are territorial, thus a trapped animal can bring unwanted attention from other animals. In the sense of rescue, I believe it's probably more done to rid the animal from it's terrain.
