Rocks certainly shares some traits with Luffy, but he is fundamentally a violent person, and more akin to an actual pirate and criminal than Luffy.
That's what they TELL you, but have you actually seen anything that indicates he's so violent on paper?
He didn't want to hurt Loki initially and when the latter attacked he himself taught the kid a hard, but fair lesson. He didn't go overboard while he could've easily just killed Loki there. Just enough to show who's boss. Very similar to Luffy, who has done so multiple times, too.
Then they leveled an island - yes, an island full of scumbag monks that scammed everybody. Not a horrible act at all. A very SH thing to do.
Then we've got his fight with Harald. He clearly holds back just like Harald, because none of them wants to kill the other. Again very much like Luffy (actually a copy of his fight against Usopp).
It's very clever on Oda's part, really. Everything the manga tells you (the WG actually tells you) directly contradicts the actual things we've seen Rocks do on paper.