Blood Red Daffodils by M.P. Swinnerton

This is the story of 3 people who have been friends from childhood but jealousy creeps into the equation as they become adults both in their love lives and their businesses. The one female in the trio is the cause of the rifts in the friendship and one of the the men eventually becomes so obsessed with beating his rival that he turns to killing as his way of solving his problem. This is the way he sees he can combat his rival in business and that is by producing red daffodil flowers. He starts murdering innocent people in order to obtain the fresh blood he requires to feed his plants and this then becomes his normal way of life.