First thing is that they aren't evil. Their goals simply run counter to the goals of humans, elves, orcs, etc.
Second is that prior to the Titans arriving and shaping Azeroth, the world was already shaping itself naturally. Like earth being uninhabitable for millions of years before life finally began to take root. Obviously the cycle was fairly advanced since we know at least trolls existed prior to the Titans arrival. It was a harsh place back then, but whose to say how Azeroth would have turned out had the Titans never touched the place? I think it would have been very similar to Draenor.
Also, I think the stories we read in game about how the Old Ones held massive elemental wars amongst each other were closer to symbolic references to how harsh the climate and nature could be back then. Again, I liken this period of time to the ages of huge volcanic eruptions and tectonic shifts until a balance was finally found. The Titans saw this process as taking too long and began to shape the world to fit their views.
Third thing is that forcing Azeroth to change the way they did set the natural balance out of sync. Between the time they left and their return this out of sync/ loss of balance took the form of the Old Gods who were at this point trying to return Azeroth to the natural order they embodied, which to the Titans is Chaos.
So according to my theory, there is a bit of cosmic irony here. That the Old Gods represent natural order, the titans represent artificial order, and their clashes end in total and utter chaos. Thus, it is up to us to bring balance .
Note: Also, keep in mind that I'm not saying an Old God is good. It's hard to explain actually, I guess it's that there is a reason they do what they do and it isn't necessarily evil, it's just in their nature to return Azeroth to the natural order, according to this theory.
So any glaring holes in this theory?