Uther was self righteous which was exactly why Lothar didn’t want him as his lieutenant(instead choosing Turalyon) as he would have refused to do what was necessary against the Horde!
Uther was a Knight who received Priest Training so the Priest Training had in Lothar’s mind corrupted him. In Lothar’s mind Turalyon who had Knight Training after his Priest Training gained a perspective from the Knight Training that made him more willing to do what was necessary.
Lothar was right as Turalyon was willing to go along with the method Alleria was forced to resort to in order to get information about Sylvanas!
Turalyon while still brainwashed by the Light is not as deeply tainted as Uther is!