Because Trevor is loyal. The most prominent example is during Ending C, when Devin is in the trunk. He's offering tons of things to Trevor if he lets him go and turns on his friends. Trevor knows Devin will have to do it. He knows he's capable of following through with this deal. But Trevor continues to brush away his offers and states something along the lines of not betraying his friends.
Another is the one mentioned int he OP. If you kill Trevor, Michael will tag along. Kill Michael, Trevor wants no part of it.
Trevor may be the craziest of the three, and have some real mental issues, but loyalty is a big part of his character. Another one that just came to mind is that Trevor also mentions at some point Brad wanted to kill Michael and run the heists himself, but Trevor didn't want any part of that either.
Loyal to who? Michael and Brad?
He wanted to kill Franklin for laughing at him.
He doesn't have any friends. Wade, Ron and Chef are his drug addict little bitches scared of him.
To his friends.
Sure he was pissed at Franklin, but he didn't really do anything. You're blowing it out of proportion.
And yes, he does have friends. Wade and Ron are, it's just that they're are so afraid of him that they place themselves below him, and put Trevor on a pedestal. Trevor thinks they're losers, but they're still somewhat friends. Then the obvious are Franklin and Michael, they are friends, the only ones that aren't a couple of losers, and that he respects. Saying Trevor has no friends is a ridiculous statement. You really ought to listen to the dialogue, or have the protagonists hang out with the other protagonists. It provides a lot of background and character to them. Unless you're deaf, but don't worry, there are subtitles for you.
EDIT: Looking at your other posts, you don't like Trevor. It's no wonder why you're attempting to blow out any explanation to the OP's question. You're just being biased.
Adding to that, when you do the mission for Trevor when you steal the Cargo Plane, Trevor tells Ron he missed him, showing that Trevor cares about Ron and Wade deep down, although just loves to scare them and beat them up.