Is not required to have an opinion on past events based on nowadays standards. Historian do that all the time, they look at the history with objectiveness especially when is far away to avoid to get trigger by random deaths that happen all the time (even nowadays).
So if you want understand the history the worst thing to do is to cry because someone died. You need to understand events and why these things happened and then maybe you can learn something.
I don't think is complicated to understand, but apparently seems to be the case.
If nobody is offended then why are we here discussing about how many people have died in the colosseum? Just enjoy the fact that now you can see more as compared as what we were able to see before.
Maybe you never been there, so just pay a visit and see the magnificence of it. After all, is still around after 2000 years and pretty well conserved.