I agree with you in principle, but speaking from strictly a strategic perspective, /r/place presents a vulnerability that can be exploited without contributing what Reddit really needs to regain its status - content.
In the context of a social media war, this means we can use the space to peel off users who wouldn't have been introduced to the Fediverse otherwise, with no risk of harming ourselves or providing Reddit more than a temporary moment of attention. There's been a concerted effort to remove links to Fediverse resources in many subs, and for a great deal of redditors, they're in the dark about what the Fediverse represents and how much better it is than Reddit.
Plus it's just cool to fuck with Spez, and the Fediverse is so cool in general that I'd love to see what the effect would be if we united against a common enemy.
We also need to note the time difference. Khan lived 8 centuries ago and hitler only 1. Hitler is more fresh in our minds and his memory will fade like Khan and the many others before him. How many people know of the atrocities that Assyria committed thousands of years ago? Hitler won't be seen as nearly as evil in say 3500 because they will most likely have someone from the 3300s or 3400s who they see as evil and want to not repeat the actions of. Khan may have lived in "a time of butchers" but people then also saw his actions as atrocities. We aren't that much better than other periods of history really we just see our selves that way because that's where we live and we don't see the day to day experiences of people hundreds or even thousands of years ago and only hear about wars and battles and what empire raped the other worse.