Also Maul took on and matched Mace Windu and another Jedi at the same time. Mace, the guy who beat Palpatine, who is very clearly more powerful than Vader. Maul never lost a fair fight to Obi Wan, the first time he clearly won and then the writers had make him lose, the second time Kenobi went after a weaker person Maul cared about to distract him (yep, a Jedi did that) and the third time Maul had just spent days wandering a desert without food or water. In a fair fight, Vader has no chance. And also bear in mind Kenobi would be a lot more conflicted and thus weaker while fighting Anakin, someone he loves, than while fighting Maul, his nemesis.

"good men don't need rules, and today is not the day to find out why I have so many" vs "it's taken all this time for me to realise the laws of time are mine to command, and they will obey me!" Yeah, 11 was certainly capable of being angry but even then he works jokes into it. "No I want you to use those exact words, run away, I want you to become known as colonel runaway, I want you to go down in history as colonel runaway, I want children laughing outside your house because they've found the home of colonel runaway, and when people come to you and ask if trying to get to me through the PEOPLE I LOVE! is in any way a good idea... I want you to tell them your name" 11 got angry but even then he was angry in a more realistic way and he knew who to direct it at, 10 definitely had a dark side and at times seemed a lot less in control.

Yeah, the whole must be won in combat thing makes no sense in reality. What if the current wielder dies from something else? What if they die in battle against numerous foes at once? The moment any of those happen it can never be wielded legitimately again apparently. It sounds like something a writer thought would sound cool and didn't think about the actual ramifications of.