I am agree that after Jesus in our heart, we Will purified by Jesus, but Seem to me not over night. It is a proces when Jesus in our heart, we do not want to kill other but to love, but as a baby though live baby, It can't walk perfectly, need learn, It may fall and make mistake. But I believe when we fall in sin we need to repent again. Say for some reason a born again Christian kill other, he have to ask God forgiveness and stop killing.
When David kill uriah and Nathan confront him, and he confest.
2 Samuel 12
13 And David said unto Nathan, I have sinned against the Lord. And Nathan said unto David, The Lord also hath put away thy sin; thou shalt not die.
14 Howbeit, because by this deed thou hast given great occasion to the enemies of the Lord to blaspheme, the child also that is born unto thee shall surely die.
15 And Nathan departed unto his house. And the Lord struck the child that Uriah's wife bare unto David, and it was very sick.
16 David therefore besought God for the child; and David fasted, and went in, and lay all night upon the earth.
17 And the elders of his house arose, and went to him, to raise him up from the earth: but he would not, neither did he eat bread with them.
The way David respon is confest and fasting ask the Lord If He forgive him and not kill his son.
So I believe we need repent, regret what we do and ask forgiveness every time we sin.