I think so. In the book it said that he was forced to serve Galbortrix and Eragon is hoping to find a way to free him so I think that in the fourth book Murtagh will either turn good again or die. I don't know how but that is my guess.
Yes, Murtagh was Eragon's half-brother.
Brendan Murtagh was born in 1983.
Tim Murtagh was born in 1981.
Chris Murtagh was born in 1984.
Johnny Murtagh was born in 1970.
Andy Murtagh was born in 1949.
Elaine Murtagh was born in 1940.
John Murtagh was born in 1943.
Murtagh believed he was telling the truth. Eragon and Murtagh really are half-brothers and share the same mother. The part Murtagh was incorrect about was who Eragon's father was. Murtagh's father was Morzan, whereas Brom was the father of Eragon. Eragon was not even aware of this himself until after Brom's death.
Murtagh is certainly not evil. He is just being controlled. Galbatorix has complete power over him, and keeps Murtagh as a slave.
Murtagh willl most probably try to free himself from Galbatorix's clutches. Galbatorix may have Murtagh's Eldunari, being able to control Thorn's body and Murtagh's mind. Murtagh and Thorn could help free the green dragon egg and get it to the Varden because Murtagh may not want the dragon to be in bondage. Murtagh may also die in battle. Murtagh may kill another major character, either himself or a tool of Galbatorix. To find out more go to inheriwiki.com.
Garrett Hedlund plays murtagh in the movie eragon.