Who do you think is morally worse? Heisenberg or Saul?
My vote would be for Saul, primarily due to his coldhearted treatment of Howard (which I never quite fully grasped his reasoning behind, as Howard always seemed pretty good to him), him seemingly being on the verge of murdering an elderly Carol Burnett before catching himself (I didn't think he'd actually go through with it, but even the threat alone was a bit much), and him being perfectly at ease with stealing from the friendly dying man (and countless other marks over many, many years). And if we want to get feminists on things (which one normally loathes to do, but it adds to my argument here so I'm gonna), he technically tricked what was presumably many women to have sex with him by pretending he was Kevin Costner lol.
Personally, I can't recall ever once thinking that Walt was morally bad or evil at all. Or that he ever intentionally set out to hurt anyone (anyone who wasn't trying to hurt him or his loved ones first). Now, admittedly, due to him a lot of truly bad stuff happened (usually in an unintended butterfly effect sorta way) but, to me, Walt always had good reasoning behind his actions (save for his first bad action in choosing to cook meth). And when he did "bad" things, it almost always spawned from him being backed into a corner and being forced to act (or not act, as with the case of Jane) as a means to preserve his, his family, and/or Jesse's life (even with the Brock situation, it appeared that he was simply taking the most rational option when faced with a difficult trolly problem). And even in all these instances, we always saw remorse from him. He clearly regretted the things he was having to do. But, considering the context he found himself in, they usually had to be done.
Saul, on the other hand, appeared to almost compulsively take advantage of people for no reason other than personal greed. I can't even imagine Walt doing some of the things he'd done. Granted, his acts ultimately led to less tragic outcomes (save for the Howard situation) than Walts, but his cold willingness to hurt people rarely came with any visible regret. In fact, it was often followed by a celebration of a job well done and a happy wallowing in the fruits of his unethical behavior. Come to think of it, he also callously tried to get Walt to kill both Jesse and Badger (at different times) in Breaking Bad.
If it were just me, I wouldn't even think to compare the two, as, from my perspective, Saul is clearly the worse of the two. But I've seen quite a few people turn completely against Walt over the years (even going so far as to call him "evil" and "heartless") so I realize this may be a more difficult versus situation for others than it was for me.