Jamieson, Fausset and Brown on Isaiah 54:16

16. The workman that forms "weapons against thee" (Is 54:17) is wholly in My power, therefore thou needest not fear, having Me on thy side.

for his work--rather, "by his labor [Horsley]. "According to the exigencies of his work" [Maurer].

waster to destroy--(Is 10:5-7; 37:26, 27; 45:1-6). Desolating conquerors who use the "instruments" framed by "the smith." The repetition of the "I" implies, however, something in the latter half of the verse contrasted with the former understand it, therefore, thus: "I have in My power both him who frames arms and him who destroys them (arms)" [Rosenmuller].