Those who do not know history's mistakes are doomed to repeat them. You might be surprised at how many of the lessons learned by previous MMO devs, and even MUD devs, are still relevant now; more than once I've seen MMOs face severe issues that not only were tackled by earlier games, in some cases they were predicted by studies from the 90s about MUDs.
There is no universally agreed upon definition of griefer and griefing; that one you copied from Google is just one of the most common. The most common definition is kinda worthless for trying to get rid of griefers anyway, as it would require proving intent. And redefining an issue can have a powerful effect in convincing others; that definition of griefing I posted was instrumental for implementing in UO a zero tolerance towards griefing and griefers, as it made the managers aware that griefing directly impacted the game's bottom line, and thus served to export that kind of zero tolerance to other MMOs that were being created at the time (more about it
here).
The assassination of Lord British wasn't griefing, it was simply exploitation. Griefing was players repeatedly stealing everything newbies had even though those items were worthless; players goading others into stepping into a portal that would strand them in a tiny island until customer support could help them out; PKers repeatedly killing the same player until he ragequit; etc.
But then, of course, not everything that was driving players away was technically griefing; while a lot of it was legitimate PvP, the end result was still that over 70% of new players would leave UO within the first 60 days. It's why the dev tasked with rescuing UO, Gordon Walton, did so by making PvP effectively optional, even though he loves PvP (to the point he is currently making an MMO where PvP is at the core). You can read about how Trammel came to be
here.
Also, you can blame griefers and PKers in UO for how open PvP became so scarce in MMOs that came after that. UO clearly taught everyone that was watching the MMO market that open PvP wasn't a good match for a mass market game, and due to the cost involved in creating them MMOs for the most part need to be mass market games.