I don't think she could have become Dark willow in season 5, she did not have the experience, I mean compare the spells she was casting in Tough Love with the spells in Two to Go and Grave. And think back to the circumstances of season 6, Willow was separated from Tara for most of the year, then she gets back together with her and then a day later she is taking away, I mean that is a highly emotional state to be in, probably more powerful then if she had lost Tara at a 'random' time during season 5. Not sure if that made sense, In season 5 she would have been in a continues relationship with Tara until that point where she lost her, where as in season 6 she lost her and then had to deal with knowing what she had lost, then got her dream come true and got back together with her, and then lost her again! And it was that which allowed her to give in to the darker side of magick.