Robert conquered ?
Jaime killed King Aerys. Without a king, the old kingdom is in pure chaos very quickly.
Maester Pycelle opened the gates to King's Landing. That didn't help the old kingdom.
Ned Stark and the North played a huge role in the war. And Tywin Lannister and his gold and armies. I bet the armies from the North and from Casterling Rock were a lot bigger than Robert's Army.
All Robert did, was kill Rhaegar. And from what I've read, Rhaegar was actually a nice bloke.
Usurper. The more books I read, the more I believe that that title is actually pretty accurate. If these books were about the good and evil, I wouldn't be able to tell who is good and who is evil. They all seem bastards to me.