I would disagree with you. Tom was good at blending in and hiding what he was. Remember, when we saw him in flashback in Chamber of Secrets he had already murdered Myrtle (through the agency of the Basilisk). That was why his diary was a horcrux.

In the flashbacks we saw in Half Blood, Tom is younger than in Chamber. He is finding out about horcruxes. In Chamber, the 16 year old Tom had created his first horcrux.

In Half-Blood he seems colder and more calculated. That could easily be because he is younger and not as polished as he is by the time he's 16.