Unacceptable

I'm told that this sentence is unacceptable:

*John finally killed Mary, but it took him six months to bring it
about.

Could anyone tell me why it should be?

I don't know; it sounds okay to me, although normally I would say "John finally killed Mary, but it took him six months to do it."

The only question this raises is did the process of killing her actually take 6 months (slow poisoning, perhaps?) or did it take him six months to actually work up the nerve (as might be implied by "finally")?