Hey,

if the killer is AKF and near the 1 gen left he decied to farm, I finish the last gen and he also opens the gate and I leave throu it, does it count as a valid run?

It sounds more like a rejection, because the killer is throwing the match

I don't think that such a run would be seen as valid. Your opponents need to actually try. Kill% runs get rejected all the time for DCs or throwing the game.