It wasn't a slight risk. Poisoning anyone is never a slight risk. There are allergies, adverse reactions, drug interactions, potency variations, etc. etc. Perhaps Walt accounted for as much of that as he could, but he still poisoned a kid, just as like, a move, you know? That is the behavior of a sociopath, and is abhorred by both decent people and all reasonable criminal justice systems. There's also blowing up Gus, which (apparently) had no collateral damage, and that is totally a conceit of the show's writing, because in real life there would have been other dead innocents in that situation. The show carefully kept Walt from deliberately murdering innocent people, but this is a matter of the writers hiding the true consequences of Walt's actions from us. What of the thousands of people who used his product? What of the gang violence that arose from his drug empire? He enabled who-knows-how-many murderers and thugs with his blood money. Drug empires aren't built on niceties and goodwill, they are built on intimidation, violence, extortion, bribery, and murder. Walt did his best not to get his own hands dirty, but the blood is still on him. Now, Walt didn't go into all this with his eyes wide open. He had to learn the ropes and realize just how dangerous his new venture was, not just to himself but to everyone around him. Even once he knew the full scope of his profession, he pressed onward. These are not the actions of an ethically circumspect individual. Whatever sympathy he deserved at the start, he deserves none by the end.