she lost her friends and her children (dragons), she was betrayed by her closest advisors and rejected by the one she loves... as she said, she has no love in westeros, if she just won the battle, she wouldve been betrayed again and would have lost the throne and everything she fought for...or worse: get murdered (varys already tried).. imo in her mind it was the only thing she could have done to conquer the throne and stay! on it... she needed to spread fear to everyone including and in particular to jon and her (former) advisors, so that no one ever tried to betray her... i think shes perfectly aware of what she has done and she wont ever let those people near her, at least nowhere near as close as they were before...
i dont think she became a totally different character.. while she definitely wants to help the poor and helpless, her priority or rather obsesssion was always conqering the iron throne and ruling westeros.. she just never had to decide between ruling and helping people.. in mereen people were held as slaves and by conquering/defeating their tormentors , she was able to help those people... but also back then she was super ruthless to those who stood in her way.. think of the crucifiction, various burnings, etc... in kings landing tho, it doesnt make much of a difference to the common folk who is actually ruling them, there is no one to be freed in that sense... so how could she get the love of them? especially if someone else has a better claim to throne, is already loved by everyone and regarded as a better ruler...