How they get supermassive: I don't think anyone knows for sure. But it's not thought that these guys formed from any single star. I've heard that they may be the direct result of gas collapsing into the center of the galaxy, not necessarily having ever been a star or a group of stars first -...
Black holes are ambush predators, not hunters. They only eat things unlucky enough to get into unstable orbits ridiculously close to the black hole.
Otherwise, they act as an ordinary source of gravity, like the Sun or Jupiter but much bigger. As such, they can attract matter looking for a...