The Gemara says that עדים זוממין are not killed if the punishment that they [falsely] conspired to give to the accused person was already carried out. כאשר זמם ולא כאשר עשה.
The question is that the Torah says יד העדים תהיה בו בראשונה - it is the witnesses who carry out the death penalty. So even though we can't get them on grounds of being עדים זוממין - they should be killed for murder!!
