If she lost Kennedy in season 7? No, she didn't love Kennedy enough and I think that she would have to lose everyone she loved for her to lose it that way again. She had just learned to deal with grief.
Did she love Kennedy as much as Tara in season 8, so that she would have lost it if Kennedy died? We can't know, but I guess not, for the reasons stated above, but also because Willow had a much stonger control and understanding of magic. She knew the price. I think she might have lost it and wanted revenge (she did get somewhat dark at times in season 8) but she would probably not go all Dark Willow. Not even if it she grieved Kennedy more than she did Tara.
Btw, I can't recall the moment very well, but in the last issue of season 8, it's implied that Willow either loved Tara or Saga Vasuki more than Kennedy. She had been in denial or something, which had a part to do with the break up with Kennedy. So maybe her grief over Kennedy, even if murdered, wouldn't have made her close to as upset as when Tara died.