i think you have a good idea there.
Diablo and baal were fighting a lone tyreal. in that time they could have started to corrupt him.
But... the destuction of the worldstone caused many problems for the world of sanctuary. so perhaps it was not a matter of his last will for good, but a cleverly disguised ruse, masterminded by diablo himself.
on the diablo 3 site, there is a line: "and the heavens shall tremble". if diablo has control of tyreal then, as baal has the knowledge of tal rasha, then diablo would have the knowledge of tyreal. this would grant him access to heaven, to attack from the inside.
tyreal also said something about a final rest being deserved. that sounds to me more like a subtle death threat. the d3 home page gives a basic description telling whats going on. it says that the heroes are all either dead or mad. it doesnt say how they died. maybe tyreal/diablo killed some and couldnt find the others.
that, by the way, didnt seem very characteristic to the tyreal we know. or any angel for that matter.
i feel that there are many possibilities left for speculation on the whole "evil tyreal" subject, alot of which seem completely plausible. but they are just that, speculation.