What was your most "killed off characters" playthrough?

True but, if you want your cake and eat it too, i.e.  the cure, wanting both geth and the quarians...it can be quite an emotional trip to play this game with Bioware's evil default decisions.

Can't wait to start my Thane, Jacob and Jack free game soon (after Andromeda). 

Well now I have to go get a bunch of people killed so I can see how the different characters play out. 

Well, it will be lonely, I can tell you that. LOL

You will only have Liara, James and EDI as squadmates (and Javik if you have From Ashes).
The Normandy will be a very empty ship too.

And as pointed before, Padok Wiks is a very interesting replacement to Mordin.
The chat between Dr. Michell and Adams during the Tuchanka arc is very touching too.

And now that I'm thinking off: my Shep is a serial killer, I killed Samara and her entire family!

His playthroughs are the light side.

Here's the best scene in the Citadel dlc I've done that a couple of times.

I'm curious if anyone has tried that? I don't use mods since I play on the ps3 and I've never played Citadel after finishing the game.

No.

Ashley dies in 100% of my playthroughs.  It's not because I hate her so much as my desire to save Kaidan.  Just how it is.  I guess, because I don't play Renegade much, Mordin also always dies because in order to save him I have to kill the krogan.  (Slideshows with living rachni prove that to be true.)

Nothing is certain but I feel the slideshows are meant to suggest outcomes that were a direct result of the in-game events.  It could be that a weakened krogan were overwhelmed and destroyed by a resurgent rachni.  I tend to find it unlikely, with all the technology at their disposal, that asteroids are killing anyone (except intentionally).

In my first and "Canon" playthrough of the trilogy all those years ago, I had

Kaidan(Virmire) die in ME1

Jack(Suicide Mission-I made the mistake of listening to Miranda), Jacob(Suicide Mission-he volunteered) and Samara(Suicide Mission-I picked her as 2nd squad leader because I didn't want to show bias towards Garrus lol) die in ME2

Miranda(Killed by Kai Leng), Thane(Killed by Kai Leng), Mordin(I shot him), Tali(Committed Suicide), Legion(Sacrificed himself) and Wrex(I shot him) die in ME3

The squadmates who survived the trilogy were Ashley(My LI), EDI, Garrus, Liara, Javik, James, Zaeed, Kasumi and Grunt.

Having also played through the trilogy with everyone alive, I can honestly say that I prefer my main playthrough where squadmates were dropping like flies, it fit much better with the story Bioware were trying to tell, especially in ME3 where there was an Intergalactic War of Extinction going on.

Also, many of the missions where some squadmates are written to appear are much better without them, such as the Grissom Academy and Ardat Yakshi Monastery missions. Grissom Academy without Jack allows for the students to have some actual character development(as well as giving us one of the best Shepard quotes in the entire series if you choose to send the students to the frontlines at the end), whilst the Ardat Yakshi mission without Samara removes the worst written and most nonsensical scene in the series, Samara's Suicide.

For her to be killed, ems has to be below 2000 and taken on the beam run. Harbinger will kill her.

My first blind playthrough of DA:O wasn't too far from an "everyone dies" scenario. It just happened. I went in determined to be a power-hungry mage and didn't redo my choices for better outcomes. I didn't recruit Sten or Zevran because doing so was crazy, so Sten died in the cage and I knifed Zevran. Leliana got killed because I defiled some pottery. Wynne died in the Circle tower because apparently picking the "I will get rid of these pesky mages" line meant agreeing to kill all of them with no chance to back out. And Shale died when I wouldn't destroy the anvil of the void. Because Shale was recruited late and Oghren last, for much of the game I only had Morrigan, Alistair, and the mabari with me.

Regarding the OP, I once did a zero-persuasion, colonist/sole survivor Shepard playthrough where many of my companions died on purpose. He was a sentinel named David Dunn from Unbreakable because he always kept fighting and couldn't be broken no matter how many died around him. Wrex and Kaidan died in ME1. Mordin, Tali, Legion, Miranda, Zaeed, and the entire Cerberus crew died in ME2. Samara, Ashley, and Padok Wiks died in ME3. Of course, many other non-squad NPCs died as well. Dunn was basically a screw-up. Miranda's sister plummeted a few stories with her father because I didn't use persuasion or the interrupt, ME2 loyalty missions were intentionally botched like Thane's and Samara's (which you need persuasion to succeed at), Feros didn't survive, Kirahee died, etc. It was pretty bad.

That is a lot of mayhem! I had a world state import similar to your DAO run but I never played it out myself. It seemed like it would be lonely. I was the same with MET but I've done so many PTs, I want to change things up. I picked up ME3 after a couple years, only playing it once on release. I didn't know what the refusal option was so I wiped everyone out that time. Does that count?