No one in this scene thinks of Regina as Henry’s mother. That’s why Grumpy can talk about going to watch her lynching in front of her ten year old son. And there is also the fact that David only wants to interfere out of fear that the Queen has her magic again and others will get killed for attacking her.
Not that Regina, or anyone, has a right to defend themselves from a mob.
But here we have Henry who in an instant realizes that he may be responsible for his mom’s death. Because before this moment he was on a mission to defeat the Queen. And it never occurred to him that defeating the Queen meant killing her. The problem of never having watched all those Disney movies where the villains fall off cliffs. Including the Evil Queen.
But the thing that always gets me is that he was talking directly to Regina about wanting her defeated and she did know what that meant. And she never told him. Never asked him what he thought would happen … because she was so desperate for everything to be normal that she couldn’t bring herself to tell him the truth. And tell him that she was not going to survive the curse breaking.