Passing list into another list

Suppose I have a zero matrix Amat and a list of values ltest that contains the values of the matrix Amat for the some of its diagonal elements:

L = 10000; Amat = 0. IdentityMatrix[L]; ltest = {{-0.70710, 1329}, {-0.25, 5607}, {-0.3535, 1261}, {-0.25, 4393}, {-0.5, 4358}, {-0.3511, 1009}, {-1., 3815}, {-0.25, 3784}, {-0.133, 3667}, {-0.88, 909}}; 

Now in order to pass the second column of ltest to Amat I can do:

For[i = 1, i <= Length[ltest], i++, Amat[[ltest[[All, 2]][[i]], ltest[[All, 2]][[i]]]] = ltest[[All, 1]][[i]]] 

Is there a shorter way without using For?