Question: If a character is given a strong impression at first with endearing qualities (and assuming that it makes an impact on the reader), how far do you suppose you could blacken his/her morality before this impression wears off.Answer:
That's a difficult question to answer without seeing the manuscript.
Often, it depends on the context - how moral the other characters are. For instance, you can have a main character who is a criminal, even a murderer, but if he is still seen as morally superior to the antagonist, he can still be a hero. (For example, Macheath in The Beggar's Opera
or Artemis Fowl.)
Stories often offer moral arguments, along the lines of "in this type of situation, X is the better course of action than Y." But it's very hard to give an absolute moral principle like "X is ALWAYS the right course of action," because real life seldom works that way.
It also makes a difference if you put your character in a situation where he has little choice but to do an immoral act, or where doing the moral thing could have a worse result. For instance, most people would say it's okay to take a life of a murderer to protect an innocent victim - if there is no other choice. But there are many, many shades of grey. Obviously, if the reader sees the character make an immoral choice when there was a perfectly logical moral option open to him, that will adversely affect the reader's opinion of the character.
Fortunately, as the author, you are free to stack the evidence for and against your character so that the reader gets the impression you intend.