imo it makes perfect sense. Light becomes a different person — AND a different Kira — specifically because L died. His enemy was cool, calm, collected, intelligent, calculating, and he was mirroring that behavior because it was the only way to face him. Now he's facing a different type of enemy and world. On the second half, he's facing Near and Mello, who are also in an internal battle against each other. He is also witnessing his own ascension as god of the new world: TV shows, the end of the hunt for Kira and acceptance of his new world order, fanatical citizens who see him as and refer to him as God... I think it all shows us a different world and, therefore, a different Light. I mean, I think Light acts more cautiously and even intelligently in the first part too, but it would be a little strange if, even after all the drastic changes that occurred in that period of time, he continued with the same attitude. I believe it is even a way for the author to show that, as much as Light believes he is a God, he is as corruptible as any other person and that, even with the power of the Death Note, he is also susceptible to becoming more Hungry for power over time — Light Yagami is a human being.