I mean, the Sith are literally an immortal boogeyman that have killed trillions over multiple galactic wars, basically every Jedi was just murdered, many of them personally known to Obi-Wan, and Anakin also personally murdered a room full of children.
I'll forgive Obi-Wan for a bit of rhetorical hypocrisy.
Becoming a Sith used to be very hard to do since it’s such an exclusive club with the Rule of Two and all. Now these days all you need to do is deal in absolutes to qualify.