Gori-sen ~The type of PE teacher to die first during a panic~ - Ch. 73 - The type of PE teacher to be saved by a time traveler time leaping from the …

[Swagner]

I assume the assasin is from the future too, and future people learned about the attack and sent the andriod too, to avoid the time line changing or something. But they should probably know the result, although maybe it takes a bit of time for the change to take effect?

The rationale from the (first 2) Terminator movies was that nobody, including SkyNet, actually knew whether time travel could change the past or not; most records from before "judgement day" (the nuclear armageddon) had been lost, and SkyNet was scared enough of the possibility that messing with the past would accidentally prevent itself from being invented in the first place that it didn't send the original T-800 back until it had already lost the war in the future and was about to be shut down, and had nothing left to lose.

Hilariously, in that original continuity (every movie from Terminator 3 onward retcons at least part of this), the past couldn't be changed; SkyNet was actually built by a company that had salvaged parts from that first T-800, and so SkyNet was responsible for its own creation!