The ending explained (spoiler).

The movie does not end with the bedroom scene. The bedroom scene only sets the stage for the ending. In the bedroom scene, both Kate and Merton realize what they had done to Millie, who they both cared for while plotting to take her money, and how this had changed their relationship. Their sex was bitter-sweet to mirror this, as they both realize they had destroyed the love they once shared.

They contract for marriage where Merton wants marriage without taking any of Millie's money and Kate agrees to this if Merton can swear he is not in love with Millie's memory (which he most definitely is).

Merton does not meet Kate's marriage-without-the-money condition of swearing he is not in love with Millie's memory. Merton fell in love with Millie and realized this in his last meeting with Millie before she died.

The final scene is Merton returning to Venice. Did he return to relive memories of Millie? He is alone, which tells us he did not marry Kate. But did he take Millie's money in not marrying Kate, as the quote from Millie about "good things will happen to Merton" suggests?

The viewer decides these answers.