I am looking for a manga where the main character was reborn back and eventually discovers that he was reborn back with a help of lady demon spirit who is forcing him to kill.
In the last chapter I read, he needs to kill a dragon so that the black lady spirit won't eat his soul.
I believe you are looking for Hiraheishi wa Kako wo Yumemiru, also the answer to this question.
The manga starts out with the main character dying and being sent back in time to when he is a baby. As time goes on, he eventually meets back up with an evil spirit lady who he knew in his first life. She needs him to kill so she can gain power. At one point, they end up fighting a dragon.
