Kleomenes wanted to become something like hegemon of the koinon, as peace terms, and rewrite the constitution to make the office hereditary. At least, according to Polybios.
I honestly can't particularly condemn Aratos for his decision to turn to the Maks. In the event, it turned out to be a fairly good thing for the Achaians; Antigonos Doson wiped out the Kleomenean threat, then promptly died, and Mak influence in the Peloponnesos nearly disappeared. Losing Akrokorinthos was bad, but hardly crippling.
Where I think he was a bit of a failure was in terms of military leadership. Honestly, the Achaians shouldn't have been forced into the expedient of turning to the Maks in the first place. Sparta was practically dead in the 240s and 230s. Now, Aratos may also have been a bit precipitate in bringing Megalopolis into the koinon (thereby diluting Achaian military and political power and centralization), but it's pretty hard to tell - the sources for that are even crummier than they are for the Kleomenean War.