I don't think David is as evil of a person as everybody has thus far made him out to be. I think KA created him as a foil to the Animorphs. David like the Animorphs all lead normal lives before learning about the invasion, but how David responded is what separates him from the Animorphs. I don't think David is evil, but rather weaker than the Animorphs in determination and mental strength. When David lost his life to the yeerks, he started succumbing to selfish and easy acts.
I don't really think it's possible for anyone to proclaim that David was obviously going to turn evil or bad before reading the 21, or even 20. All of the other Animorphs have moments of weakness, Rachel's aggressiveness, Marco's ruthlessness, Jake's cold practicality, but in the end they "do the right thing" and emerge a hero. On the other hand, David who also started to face such decisions that showed his true characteristics chose to do the easy and selfish thing, rather than the right thing.
I guess, what I'm trying to say that to me, I feel like it's near impossible to know for sure that David would have turned out bad, but there were definately signs pointing to it when he made decisions, and its the accumulation of these decisions that made him more progressively "bad"