Why would nature produce something like the dinosaurs?
I wouldn't look at it that way. Nature didn't "produce" it. It evolved into what it was because it succeeding in filling a niche. There is some debate about how predatory it was but I tend to think it was an active predator as you suggest. What is the real difference if 20 lions kill 20 wildebeest or one T rex takes out one triceratops.
They survived and reproduced. The end. Anyone looking for further or deeper meaning will be disappointed.
Because they were capable of surviving back then. There must have been quite a bit of food running about.
Nature did not produce dinosaurs in the way you imagine. Evolution simply does not work like that. If something changes in a species it gets put through the testing process that is natural selection. Dinosaurs were around for a very long time and they did not go extinct through natural selection but through a major catastrophic environmental event. They clearly passed natural selection's testing.
Nature abhors a vacuum.
T-rex had one of the most powerful olfactory systems nature ever produced. It likely was an opportunistic hunter who scavenged on the regular.
I suspect they were much better suited than you give them credit for given dinosaurs existed for about 180 million years.