Well, I wouldn't think that every single one is evil. Evil is too broad. However, there are hints towards them being one of the many new enemies in Halo 4, as well as them being the creators of the original strain of the Flood. Again, hints.
THERE IDEA TO GIVE AWAY THE MANTLE WAS OKAY BUT THE FACT THAT THEY KILLED UNWORTHY SPECIMENS IS KINDA CRUEL. aND RERLEASING THE FLOOD ON THAT SPECIES SO THAT THEY DIED WAS ALSO REALLY CRUEL ESPECIALLY CAUSE THE TOLD THE FLOOD NOT TOO INFECT HUMANS. BEFORE THE FORERUNNERS WHIPED OUT THE ANCIENT CIVILIZATION