My FI is a wonderful, caring, smart man and I am so incredibly lucky. Unfortunately, his BM is not.The FI and I have been together for 3 years and this "friend" has never had and issue with our relationship and he didn't have anything negative to say when FI told him he was purposing to me. Now we're getting married in 4 months and this guy is the best man. On multiple occasions throughout our engagement this guy has verbally harassed me, told lies about me to FI and finally came out with his feelings and told me that marrying me would be my FI biggest mistake. He followed that up by saying that my FI has never had good taste in women and never gave any actual reasons for his concern. He come across as very threatened by our upcoming marriage and when I told him that he insisted that they would always be friends and I had no say in that. FI has talked to him on multiple occasions telling him he must respect me, we are becoming one and that if he wants to be in the wedding he cant act this way. After each talk the best man promised he would try harder but nothing has changed. My FI is so stressed out about it and feels like he has to take him out of the wedding knowing that could be the end of the friendship. This guy just seems so immature and can't appreciate how important our wedding and marriage is to us. Advice from someone dealing with the same thing would be greatly appreciated.