What kind of recommendation do you give the children's series Wicked History?

I have seen a few individual reviews of specific books in the series. They use the vehicle of biography to present history.

I am curious about the overall perspective and feel of the series. Every biography picks a specific ’wicked’ word to splash across the cover: i.e. Persecutor, Vicious, Arrogant, etc. and I wonder how the focus on wickedness detracts from a balanced historical perspective. I am interested in what to be aware of and what I may need to balance with other information. Maybe it is best to look into only one or two of the books (if any).

If you have any other historical resources that you can recommend for children aged 9 and reading at an upper elementary school level I would appreciate it.