Why does the movie SE7EN suck?

SEVEN sucks because the motivations of the characters are suddenly rearranged to fit a weak ending.
The way the movie ends is that Kevin Spacey (his character--and the other actor's character whose names I don't remember) cuts off Gwyneth Paltrow's head and mails it to an empty field in time for Spacey, Pitt and Freeman to receive it. Pitt freaks out, becoming Wrath, and shoots Spacey who claims that he had become Envy. Pitt is taken away in a police car and Freeman does not retire as he planned.

This is a really weak ending, and I think the screenwriter and director know it. But that's beside the point.

The reason why this ending is bad is that it goes against everything that leads up to it. It doesn't resolve what we've learned about the characters: this in itself is not bad, but what Seven does is force its characters to deliver an ending that doesn't belong to them. Yes, Spacey is confronted by Pitt/Freeman and is "forced to change his plans," but he also is suddenly overwhelmed with feelings for Paltrow that he must cut off her head so he can become the vicitm of Pitt's wrath.
Having any sudden feelings for anyone is not within the Spacey character's rules: you can't tell us that this man would attend to someone (SLOTH) tied to a bed for a year and plan each punishment for days or weeks, yet leave a sin available so he can claim it for himself? This is not being true to the character that we learned about for the previous two hours. The "plan" was that he had not plan? Again, you're going to tie a guy to a bed for a year and plan to great detail 5 of 7 deadly sins and just improvise the last two? The second you hear Spacey say that he is ENVY, you should be screaming at the screen, "NO!!!"

I can tell you how you can fix the ending, but I don't live in Hollywood and I don't make movies. I can only tell you when they suck. That is my only power.