Revenge
Disclaimer: I don't own!
The Source wanted personal revenge against the Charmed Ones and the one demon that would have been his successor, Cole Turner. The witch, the one who he considered to be useless powers somehow wrapper her fingers around Cole.
He wanted revenge and he'll make sure that Cole would get the taste of revenge when he dies.
The Source decided that Cole Turner would be his successor. He wanted to personally see how Cole would balance his human side.
And that the Source knew that he will win. Cole will lose his love and life in the process. For he is half demon, and power is always tempting, his evil side will win. And his witch won't like that at all.
And then the Source will get his revenge. One way or another.