The movie wasn't really about anything, stuff just happened
Ronan did not want the stone he was getting the orb for Thanos, nobody knew what it was,
There's no logical reason why he would be working for thanos, when he has his own army, he attacked the planet he wanted to kill anyway without thanos, and the the gem basically did nothing by serve as a plot device for him to be beaten
he already had super strength to begin with
once I again the movie was good for what it was, but its not some ground breaking masterpiece, its a good summer movie in the vain of independence day when it first came out
Thanos and The Collector were the only 2 that knew what it was. Thanos basically hired Ronan as a favor by the bigger king, in the traditional emperors way (do this for me, and when I storm, you will be left alone). Once Ronan found out what it was, he was basically like "Fukk Thanos".