Fuck the baby. Baby lives are worth dick compared to adult lives. The point is that Walt is an anti-hero, not a villain. You can say that poisoning that kid is unforgivable, but Walt was looking at a situation where he knew both Jesse and Walt's family were almost certain to be murdered if Gus was allowed to live. He couldn't convince Jesse of this because Jesse had blinders on. What's he supposed to do in that situation? Take no action in the name of some kind of morality that gets good people killed and lets Gus expand his network of evil, or take whatever extreme steps might be necessary to get Jesse thinking the right way? In this case, Walt had to frame Gus for a crime in order to get Jesse to see the threat right in front of his nose.
Anti-heroes are about to what extent we're willing to accept people who take questionable means to to an end. The thing that makes them so interesting is whether or not we can say that what they did is actually "wrong."