A thick cloth - Probably, a cloth or mat placed between the head and the upper part of the bedstead, which in Egypt and Assyria was often so shaped that pillows (in our sense) were unnecessary.
The objection that Elisha is involved in the guilt of having suggested the deed, has no real force or value. Hazael was no more obliged to murder Benhadad because a prophet announced to him that he would one day be king of Syria, than David was obliged to murder Saul because another prophet anointed him king in Saul’s room 1 Samuel 16:1-13.