Patur– “It is also clear from the afforementioned Gemara in Shabbos that the dye of the chilazon is better when the chilazon is alive, and therefore people would try not to kill it when extracting the dye. The Murex Trunculus dye starts deteriorating several hours after its death which would make completely unnecessary to try to keep it alive.”
Very simple. Look in shabbos 75a once again. ??? ??? ????? ????? ?? which means that you are dyeing it after it is already dead.