Here's the breakdown:
* Smaug's initial plan: He's been living comfortably on his hoard, content with his wealth and power. He doesn't actively target Lake-town for destruction, he simply sees them as annoying inhabitants who live near his treasure.
* Bard's actions: Bard, desperate to save his people from Smaug's attacks, makes a bold move: he shoots an arrow that hits Smaug in a vulnerable spot, a bare patch on his chest.
* Smaug's reaction: Smaug, enraged by the unexpected pain, feels betrayed and attacked in his own home. He takes his anger out on the nearest target - Lake-town.
* Lack of strategy: Smaug's attack on Lake-town is driven by immediate rage and a need to retaliate, not a calculated plan to eliminate a threat.
In short, Smaug's attack on Lake-town is a knee-jerk reaction, fueled by pride, anger, and a desire to punish those who dared to challenge him.