I'm personally popping into Season 4 right after the incident with Cole and the Hollow to tell them Cole has accidentally become the Source but Cole's human side is still good.

If the sisters had learned immediately that was an after effect of the Hollow incident they may have been more inclined to at least try and contain and depower Cole. By the time they naturally learn he's too far gone and has been messing with them too much to trust.

I get there's other deaths and big moments but Cole's story is the one that still breaks my heart. Especially knowing his human side was trying for so long but was no match for Evil when it was in him.