为所欲为 almost always carries a negative connotation — implying someone acts selfishly, arrogantly, or without regard for rules. It's rarely used positively, unlike the English 'do as you please.'
grammar
This is a classical Chinese construction. 所 + verb creates a noun phrase ('that which is verbed'), so 所欲为 = 'that which one desires to do.' The outer 为 means 'to do.'