My last run left with a dark hole in my soul for a while...I was as renegade as you can get, a racist too, and wasn't enjoying it much. So I decided to rush thru ME3 and see what low war assets would be like.
The sadness started with Tali elegantly threw herself off a cliff, then I got an email from Tuchanka saying a huge bomb had exploded and there was a huge death toll. Then the breeder Rachni went crazy and killed a bunch of ppl at the Crucible project.
I headed over to Sanctuary only for Miranda, Shep's ex, to die in a really touching scene. Ash walked away tactfully and Shep and Miranda had a little moment together, it was really well done.
Then I'd skipped Grissom Academy, so found a datapad with Jack on it at Cerberus HQ. She was being tortured, blood was all over the walls, and my Shep realised that in not going to their aid his old friend had been taken. He then had to kill her a little while later.
I get to the beam run and totally forgot what happened with low EMS. I took Shep's girlfriend Ash and bestie Javik, only to watch them get squished by a tank and nuked by a beam. The look on Shep's face was awful, he was a jerk but he didn't deserve that. The whole low EMS thing is distinctly more depressing.
He didn't save Anderson, who got shot in the head by TIM, so had to sit there for the whole time in silence. I legit thought the audio had broken. 
Then, to reflect his mood, he told the kid to bugger off and shot at him. The Reapers won. All was lost and everyone died.
By the end I had a lump in my throat and was emotionally exhausted by it, but looking back it was kind of great. It showed me there is diversity in ME3, you can do different things, and my Shep got his commupence. He reaped what he had sowed, and in it's terribleness it was quite powerful.
I'm glad I did it actually. It was sad, but good sad, if you know what I mean.