What if we mess it up and it comes tumbling toward earth?

This is the sort of thing we would verify the calculations of, many times, and consider problems ahead of time.

We lack the ability to move a large enough asteroid to do any serious damage.

Not yet, but perhaps in the near future. The National Space Society has a proposal up for capturing 99942 Apophis via gravitational slingshot, which would be fairly damaging if we managed to crash it.

it would likely burn up on entry