In Breaking bad, perhaps one of the most dreadful things that Walt has done is poisoning the little kid. Ever since the incident happened it has surfaced up many times, but it's still not clear how Walt feels about the whole things. Is he really sorry for what he did?
As a loving father of a son, Walt without doubt hates himself for involving/using kid in his "war".
But that's just it, he doesn't feel sorry for doing it at all, mostly because from his perspective his confidence/ego (about poison dosage) is so high that he doesn't feel like he put that kids life in danger at all.
It is like pessimist/optimist point of view, to Jesse it looks like he almost killed Brock, Walt however thinks that he merely gave this kid a simple cold.
If Walt could see it from Jesse's perspective, he would feel sorry and would hate himself even more... but since he doesn't, he would do it all over again. To Walt Brock was justified mean to a goal (Walt's power).
Also in season 5 episode 13 when Walt is rushing to the desert in his car he tries to explain to Jesse on the phone why he poisoned him. He says that he knew exactly how much "poison" to put...but you could hear doubt in his voice, like he was just guessing being led by his ego.