Just today I "revived" my very first character, and did the DB questline, which I've never even considered before.
After that I decided to massacre Whiterun, starting with Heimskr. I killed every single mortal character in the outside area and the barracks next to the main gate. Dragonsreach would have equalled quick suicide, so I left the hill alone.
Instead I went to the other unclearable place: Jorrvaskr. Now, since half of the Companions are essential (I hadn't even started the questline), I knew I was committing suicide.
In the thirty or so minutes that I survived (I got two level ups to help me regain health while I simply ran around the mead hall) I did kill all of the killables. But then I made a mistake, and left the building. Next thing I knew was a bunch of Whiterun Guards joining the fight. :S
I can hardly believe I did that... that character is now well beyond redemption.
At least I got killed first by Vilkas and then (after I let it load the latest autosave) by Farkas, and not some idiot Grey-Mane.
I've used that fight, and ones like it to see how OP some of my builds are. Save first, and you can go back like it never happened, but especially with the Essentials, it's like spinning plates to keep them all knelt while you kill off the ones you can. Especially in Jorvaskr, there's just so many Invincibles in there, you can't win any meaningful victory beyond killing the weakest of them, but it's a good gauge of how Badass you really are.
As for guards, the best place hands down to compare your worldbeater to them is Castle Dour in Solitude. Depending on which quests you've already completed, you can get a literally unlimited supply of Imperial (Light) Soldiers, Solitude Guards, and Penitus Occulatus coming out of 3 doors. They aren't essential (There's plenty of them, too,) but kill all you want, they'll make more. I've only been able to last indefinitely with one Build (BAMFO) with Illusion, and Conjuration mastered. I didn't do any of the killing, just started off with Mayhem, Poofed out of sight, and topped off the Dremora Lords (x2) and Frenzy whenever one got killed, or timed out. I ran out of popcorn pretty quickly, and at some point, the bodies were literally drifted higher than my (Altmer) character's height against the walls (Particularly between the Archery Targets.) Also my personal record for highest Bounty, I stopped somewhere in the hundreds of thousands.
The best time to do this is right after the Wedding (DBH) when the POs are out in force. Literally the largest
Mêlée I've ever been able to create without Console Codes was when I popped off The Ritual to add all the bodies to the fight for a mad minute. (Actually, a bit longer, because the Frame Rate slowed to a crawl trying to process everyone. Some of them just stood there frozen, because I overtaxed the Processor that much.) But that wasn't the most Evil thing I've done in Skyrim.