Would she have killed Spade if he had went...

Would she have killed Spade if he had gone instead of Archer? Probably.

She might have tried...

I somewhat doubt that she would have managed, though. I think Spade was sharper than Archer was. I don't say Archer was a dope, but Spade, I think, would have managed to see the double-cross a little faster. He certainly wouldn't have fallen for her so hard and fast that she'd have gotten the drop on him. We see throughout the film that Sam is cautious enough and sees through her seductions. He does like her, but she never really gets the upper hand.

So, I'm sure she would have tried to move against Spade, but the resourceful, sharp, keen-sensed Sam would have beaten her to the punch.