The Bible does not offer a comprehensive enough description of Behemoth for anyone to do anything other than guess at what it might mean.
Likely, it's just a mythological creature, with no more basis in reality than the Kraken or Medusa.
This 'explanation' still makes no sense. You realize that plants are as alive as you or I, right? So eating plants in the 'Garden' would've still killed plenty of things, including the plants themselves (of course) and thousands of micro-organisms.
...What? Why would an all-knowing, completely omnipotent deity wit the ability to speak things into existence ever get 'mad'? You're anthropomorphizing God (which is a form of blasphemy).
o.O
This is just batty, dude. There is no evidence that the earliest homo sapiens (appearing some 200,000 years ago) were super-intelligent, or had the necessary facilities/instruments for conducting embryonic genetic research. You're making a very, very extraordinary claim - you need to provide some very strong evidence to support it.
There was no 'Ark' or 'Global Flood'; these are mythological allegories.
That's the answer to your puzzle.
Positing extra unusual claims in order to make sense of a preconception is not good science.
Again, you're positing extra, unusual and unsupported claims to make sense of your preconception (God made everything insantly 2,000 years ago and it was all perfect & peaceful). The most logical answer to your puzzle is that God did not conjure tyrannosaurs into existence and that they evolved, over time, into highly efficient carnivorous predators.
You're right - that enormous jaw full of razor-sharp teeth is not of much use to an animal that grazes on foliage. It is very useful, however, to an animal that runs down prey and seizes them in it's mouth. The size of the tyrannosaur's brain is not conclusively known, but it likely wasn't 'itty bitty' considering the size of the animal's cranium. It's body was also not insufficient to support the head:

Finally, the arms of the tyrannosaur were likely only used to allow it to hook onto larger herbivores it was mauling (longer arms would not have been a considerable advantage; it's jaws did most of the dirty work).
You need to get informed. There are plenty of Christian sources on the web where you can find out more about the natural world; I encourage you to seek them out.
The Bible is a wonderful piece of mythological literature, but it should be recognized as just a piece of mythological literature.

Just because the Egyptians had the competency to build the Pyramids it doesn't prove that the Israelites were equally competent.