Jamieson, Fausset and Brown on 2 Kings 8:15

15. took a thick cloth, &c.--a coverlet. In the East, this article of bedding is generally a thick quilt of wool or cotton, so that, with its great weight, when steeped in water, it would be a fit instrument for accomplishing the murderous purpose, without leaving any marks of violence. It has been supposed by many doubtful that Hazael purposely murdered the king. But it is common for Eastern people to sleep with their faces covered with a mosquito net; and, in some cases of fever, they dampen the bedclothes. Hazael, aware of those chilling remedies being usually resorted to, might have, with an honest intention, spread a refreshing cover over him. The rapid occurrence of the king's death and immediate burial were favorable to his instant elevation to the throne.

2Ki 8:16-23. Jehoram's Wicked Reign.