Doctor Who: Every Doctor's Darkest Moment

In The Curse of Fenric, it was revealed that the Doctor and Ace were brought together by the titular evil being. While goading Fenric, the Doctor claims that he would never have travelled with Ace if she hadn't been one of his "wolves".

The Doctor's explanation for his cruelty is that he had to break Ace's faith in him to defeat Fenric, which was why he was so cruel to her. He's obviously telling the truth as the Seventh Doctor clearly loves Ace.

However, the Doctor always knew that he would have to betray Ace at some point, and spent their time together building her faith and trust in him so that he could cruelly break it at the correct moment.

As Sylvester McCoy might say, it's a prrrretty darrrk way to treat your companion.