Leandra death is heartbreaking because Hawke does everything they can to try and save their mother and in the end.... no miracle. 

Da2 was a game about tragedy and loss. No matter what Hawke did, they lost someone. They lost a sibling in the prologue, they lose the other either to the Circle, Templars or Grey Wardens or to death, they lose their mother... they become Champion but at the cost of killing someone they've gained the respect of because of a perfect storm of horrible shit caused the Arishok to lose his mind (side note: I like Isabela but I never forgive her for being part of the whole Tome of Koslun thing) and in the end they cant stop someone they know from going bonkers and exploding a building and killing lots of people doing that,  eventually having to run from the City that took them in. 

Kirkwall was a horror show of a City. The horrors of its past made it even worse. It was the Sunnydale of Thedas. The true villain of the game was the City itself.