Was Eru a Sadist?

[Bombadil]

Was middle-earth just a chess board for Eru? Creating the white pieces and the black pieces(unintentionally, or intentionally perhaps) for various checkmates thorughout the region?

I'm not very familiar with how much Iluvatar had in terms of power after the creation of middle-earth, so correct me if Im wrong. This thinking came to me while re-reading appendix B as it stated:

It was afterwards said that they came out of the far west and were messengers sent to contest the power of sauron, and to unite all those who had the will to resist them; but they were forbidden to match his power with power..."

Now im not sure on the origin of the istari and if they were sent by Eru himself, or the reasons for not allowing them to match sauron's power, but it almost seemed to me like it would ruin the fun if Sauron was so easily matched.

Other things to note are his gift/curse to elves of immortality and his gift/curse to men of mortality. Was he teasing each race by showing the potential of both sides of life?

And finally, One could say that Eru represents Tolkien, because he created Arda - and we all know Tolkien loves to write tragically.

And with no further ado, im open this topic for discussion or people putting me in my place because Im utterly wrong about something!