Well, Freeza knew he was the most powerful being in Namek, being the emperor of the universe and all. Plus, he also wanted to make Vegeta, Gohan and Kuririn suffer from stealing his chance at immortality, and give them a painful death. He also kept bragging that his weaker forms were a means for him to keep his immense power under control, since he himself can't do it in his true (final form), and there you have it. Legitimate reasons to no use the full power from the start - his change from 2nd to final form was his sadist side showing off, he already had the upper hand at that point.