I don't get it.
 

In the game billy gets eaten by a t-rex when he had time to doge but Jess can doge a t-rex?Why?!?

Because story-telling.

Billy was scheduled to recieve a karmic death for his face heel turn.

Jess was in her path to redeem her self in her father's eyes, so she gets extempted from death.

Also, the situations are different.

And Telltale doesn't give you a button prompt to save Billy.

the rex thought William looked like a better snack so it tried harder?

In the movie two kids are able to stay alive by themselves while being hunted by raptors but a man with a gun can not! WHY?!?

Because only one of them was a clever girl... She was busy balancing her checkbook.. So the kids were dealing with the average girls.

Best. Explanation. Ever.

I wouldn't mind volunteering for a community moderator how do I?

You follow the rules.. help people when you can.. and get invited by TTG to help by becoming a moderator.

I helped a lot of people with answers does this count or no? well a few people I can't lie lol

It's a literary device called poetic justice.

I can think of an alternative death for Billy that might make more sense:

He goes for the Barbasol can, the T-Rex following right behind, grabs it, and jumps into the ocean, the T-Rex narrowly missing him with a bite. However, just as he jumps, the Mosasaur (which was successfully released by Sorkin, right? I don't think they ever re-contained it.) surfaces, and he falls right into its waiting jaws :D Demented, effective, would look cool, and wouldn't make Billy look so stupid.

Billy noticed that the rex looked away from his direction and took a chance. Blind hope more than being stupid.

Also, the Mosasaur was contained. That's what the door Gerry shot was for.

Ah, you're right, I forgot about the door. Scrap my idea then. (Although I do think a logical person, even in the heat of the moment, would take the plunge)

And when, during that entire episode, did Billy show himself to be logical? I think logic and reason took a dive after Oscar died and he found D-Caf trussed up and filled with Troodon eggs.

That's true, Billy was in emotional turmoil and was making some very rash decisions. Even so, I think there's a point where instinct would kick in. As a mercenary, Billy shouldn't be someone to freeze up in the face of danger. I'm not bashing the end of the game; I mean, who knows what the water would have done to the embryos? Maybe that's what was on his mind.

This ending would've been so cool! Although there is that matter of closing the gate at the end of the Mosasaur scene, but maybe they could've ended that section another way to free things up for your suggestion.

Not that I'm complaining though, because I rather liked the ending how it is.

It's been estimated that rather athletic people could outrun trex

I can't believe I'm about to do this but it's nerd time. OK, talking strictly from a Jurassic Park canon perspective (I'm not talking about current palaeoecological theories or the like, only the rules set down by the franchise) in the first movie Hammond says that they clocked the t-rex going 35 miles per hour. Usain Bolt has been clocked running at 27 miles per hour at his fastest.

So, from a pure Jurassic Park canon perspective a person shouldn't be able to outrun a t-rex. How fast did the animals really run? I'll let smarter people than myself debate that.