Is it normal that i am in love with my married best friend?

I met these two guys at work named Kevin and Daniel (not their real names) in 2008. Kevin was very outspoken and cocky as Hell. He had this arrogance about him that I found appealing. His friend was Daniel and he was shy and more of a loner type. I was deeply attracted to him and he showed some interest in me as well, he was very cordial and always spoke to me. I pursued Kevin, even inviting him into my home because he was homeless. Kevin was on drugs as was Daniel. I later found out Daniel was married but was separated from his wife. I had a relationship with Kevin for two years. During this time, Daniel was homeless. He was living in a shelter while his good friend Kevin was robbing me blind. He stole numerous things from me including my old wedding rings. Daniel finally got the help he needed. He moved down to a rehab facility and was finally drug free for a while. Kevin finally moved from my home and entered a drug rehab facility as well, but he was still using drugs. How did I know? Daniel had relapsed and was using drugs with him again.
Daniel always was attracted to me even though he was married. He was not on good terms with his wife (July 2010). I let him stay with me sometimes while he was also staying in a bunk house. He and I got very close. I was still “seeing” Kevin although our relationship was over. Daniel was still using drugs until he finally decided enough is enough. He entered a rehab program through my job and he moved into their facility in October 2010. We would practically see each other every day. He started getting closer with his wife again. She was jealous of our friendship and asked her husband not to talk to me anymore. He didn’t listen and we got closer and closer. We even had sex one time and we agreed to not do it again because it was wrong. He finally finished the program and moved into his own condo which is right around the corner from me. We continued to see each other every day and I would even spend the night over at this condo from time to time. This went on for months. His wife was so upset he was spending all his time with me but he explained to her he needed me around because I kept him drug free. He recently went back home to his wife and family for two weeks. While there, he would call me and tell me he missed me and he couldn’t wait to come back home. I used to do anything and everything for this man. I love him and I know he loves me too but he is still married after he swore to me he and his wife would never get back together. He came back with a new wedding band and he is trying to pick up with me where we left off. I am so torn about what to do. I just wish something bad would happen to his wife or he gets back on drugs again and she leaves him for good this time. Then maybe I will finally have him to myself.
Is it normal for me to be in love with my so-called married best friend? Is he at fault as much as I am? Please give me some advice.