I'm one of these persons that liked them both. Keith helped raise Lucas while noone else offered Karen a helping hand. He was always there for Lucas when he needed him, and he was always there for Karen. Some may say he was following her around being all pathetic, but I just saw it as him being in love with Karen, and wanting to help them out. He was really important to them - and at multiple occasions he gave up his own life because he wanted to help Lucas / Karen, or even some of Lucas' friends. And I was kind of glad when Karen started to see him as more of a friend aswell. I truly believe they belonged together.
That said, I wasn't that sad when he died. Sure, it was a shock, but I felt the worst for Karen who lost Keith. I think I really liked them as a couple, but Keith as an individual character was not one my favorites.
I liked Andy, and I like that he ended up with Karen, but I always thought she kind of ''settled'' with him since the love of her life was no longer around. She loved Andy, but I don't think she could ever feel the same for him as she felt for Keith. And I guess Andy knew that. Plus he was good towards Lucas.