Why King was needed as a decoy to grab the attention of elder centipede? For grabbing the attention of elder centipede,


   and killing him, King was need as a decoy for Saitama to kill him. 


Why King was needed as a decoy to grab the attention of elder centipede?
Actually, it was not King himself that was needed. In Chapter 82, you'll read the following dialogue: 


  They say Centichoro cooperates with the Monster Association so he can get revenge against Blast...


Remember that it was not King himself that made Centichoro go after him but rather, it was after he mentioned Blast. King said that he brought Blast with him and after taunting Centichoro that he was too afraid to face Blast, Centichoro, presumably angered, went after him, thinking Blast was with him. 

To conclude, King himself was not specifically needed for the task. It was his lie that Blast was there that distracted Centichoro since Centichoro apparently wants to get revenge upon Blast. Centichoro probably would not have wanted to miss the chance to get even so after hearing that Blast was there, he charged towards him. Saitama used Centichoro's hatred towards Blast against him, resulting to his defeat.