Why did Batman attack the Predator?

Predator saved him, more or less, from Alien.
So why did Batman throw a batarang on the Predator?
It seems to me that it doesn't make any sense.
Predator would have probably attacked Bats anyway, but why did Batman reacted the way he did - attacking at random?
And no, don't tell me it was because he was scared/surprised to sa an alien life form.

Ko to tamo peva (1980)
Excalibur (1981)
Terminator 2: Judgment Day (1991)