Such a wonderful topic. Kudos to the brillent and contraversal question. Here's my opinion. Tess was probably most complicated character on the show. Nature vs. nuture aside she made a horrible mistake.(I love Alex; and would in no way demean his death) but I do believe it was an accident. An accident caused by her taking away his free will. Which is what I think was her biggest sin. The writer's beginning concept was to have a girl to disrupte the dreamer couple. That was her entire purpose, but since they knew that no "ordinary" girl would cause Max to stray the used to "Alien" thing to rope him in. This made her the perfect antagonist.
As the show went on they began expirementing with the dynamic. Sparks flew between Kyle and Tess ,but instead of embracing that they brought Future Max in to once again get in between Max and Liz. But that wasn't enough. Instead they reasoned that the furthur they pushed our dreamers apart the more rewarding it would be when they got back together. However in order to do this they had to defile the characters we know and love. They made them do things you never would have thought. All because they were trying to add drama to keep it on the air. Unfortunately they twisted things in such a fashion that no one could see it being okay again.
Max and Tess sleeping together was horrible ,but I blamed Liz just as much as I blamed Max. She basically pushed Max into Tess' arms. The victum in this aspect is Tess, who thought that she had finally found some one to love her. Max slept with Tess knowing he didn't feel the same.
How many women have you sympathized with who were in the same situation?
It's after that the writer's decided to kill off Alex due to Colin's heatic work schedule. Who would do it? What would boost ratings? A betrayal! So they looked for someone to take the flack. Tess. She was used as a stand in girlfriend since the beginning. Tess was an interesting character and there was so many ways they could have taken her. Instead they switched teams half wya through and made her evil even though they had already portrayed her to be caring and devoted.
So opps she is the ultimate evil. Now everyone has someone to blame for all their problem. Liz and Max didn't break up because of Tess. One woman can not tear up a happy couple. If she breaks them up they obviously had other problems that just escaladed it. Max and Liz broke up because they didn't trust each other. They didn't believe in the love.
Later Liz blames Tess for why her and Max have problems even after she flew off to Antar. So the writers once again need to have someone to use as a pitfall for the dreamers relationship. Insert Tess. She comes back even scrapper than before and the writers scream redemption and expect us to jump on the bandwagon.
After Tess' death Liz speaks about how perfect everything was now. Huh? If that what it takes to have a good relationship with your boyfriend maybe you need to look a little deeper ,and get to the root of the problem.
Tess could have been a character just as complex, lovable, and indepth if the writer's would have just stuck with their thoughts mid season of season two. Instead they chose a lees flattering and uncreative version.
I am the type of person who loves bad turns good stories, but Tess was never allowed the chance to show that she could be more than just one dimensional. Which is why I thank my lucky stars for fan fiction writers. They right so many of the wrongs presented in the show.
Here's my two cents for good measure. 
I mean how can you be fond of someone who was thrown into a plot just to cause s**t for your favorite couple 
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