If Parameshvara is svatantra karta as per Vedanta, and if everything pre decided as per lords will, whatever we're doing is God doing actually, so is he the Cause of Evil
You're talking about the well known "Problem of Evil" in Western Philosophy.
This is discussed in detail in the Vedanta Sutras, in particular in the sutra वैषम्यनैर्घृण्ये न, सापेक्षत्वात्, तथा हि दर्शयति. The basic position is that it is not possible to blame God for evil on account of partiality or cruelty because He is an impartial observer and facilitator of events in accordance with past Karma of the individual. For the Ramanuja position on this Sutra, see here. For the position of Adi Sankara Bhagavatpada, see here.
One may argue as indeed others have that this merely pushes the causal agent of the evil to a previous birth and by extension to the very first birth. However this doesn't fit into Vedanta because the number of past lifetimes is not finite and karma is beginning-less axiomatically. If you accept this axiom as truth then it becomes impossible to find an answer that is satisfactory because it challenges our notions of causality (not too unlike speed of light, time dilation and other relativistic physics).
In Visistadvaita at least it is commonly thought that we as mere mortals are incapable of understanding the answer.