Granted, he did try to kill The Boss and Julius, but only because The Boss was after him in the first place.
But why?
Dex was the brains of TTSS and a decent guy. So he dropped his flags and got a job! Why is that so horrible? I don't know, maybe I'm just not getting it but I never thought Dex was a bad guy.
...although I do enjoy assaulting his zombie corpse over and over.
Because the Boss, for all intents and purposes,
hates Dex. He wasn't loyal to the crew and, as you said, tried to have the Boss and Julius killed. As all the four games has taught us: trying to kill the Boss and/or messing with his/her crew grants you two things: (1) a place
very high on the Boss's shit list and (2) a very severe case of being dead.
It's not so much that we players hate Dex, it's that the Boss hated Dex and we fully expected him/her to exact revenge on Dex in
Saints Row 2. Instead, Dex was forgotten about for two games and only appeared as a random event on the map in
Gat Out Of Hell. For some, it's a lackluster closure for a revenge plot that had its roots in the very first game.
EDIT: In the words of Boss, "If you wanted to be a killer with a conscience, write a book like King,
but you should never have come after me!" The bolded basically sums it all up. Had Dex simply dropped his flag and that was that, then the Boss would have had no problem with him. Dex decided to go after the Boss, which was the problem.