The T-800 (sent in 1984) was sent to kill Sarah Connor but it was trying kill Kyle Reese instead.
Why is that?
Did I miss this difference being explained?
The T-800 sent back in Genisys is initially sent to kill Sarah Connor but is immediately disabled by Pops and Sarah on arrival at the Observatory.
They load it in their truck/van and take it with them when saving Reese from the T-1000.
As they are attacked in the abandoned factory by the T-1000 (after it tracks them down) the T-1000 drops some of itself (the special metal) onto the T-800 which repairs and reprograms the T-800 to attack Reese while it kills Sarah.
A short struggle ensues and Reese blows the T-800's head off.