Terminator Salvation

I dont get it! Why did Sam Worthington has to die in the end!

I bet he would have been a huge benefit for mankind if they kept 

him. Another thing that I cannot understand is why did the 

engineers who designed Sam (guy who played human robot) put 

a real heart in his body! I do not think that was a logical way to 

design him. I also wished to see more robots in the movie, 

however, I was only able to see a couple of them. They were 

depicted in a realistic way though. They were TOTALLY 

believable.