It was, in all honesty, very probably the only act of defiance he committed in his entire life. However, he liked to believe that the magnitude of this transgression would make up for his years of servitude and sycophancy. This was an act to be remembered for. This was an act to die for.
At first, he thought just to steal the locket away, but decided to go the more dramatic route. A vacant basin would be easily spotted, easily understood. If, on the other hand, he were to replace the locket with one of his own, it would prolong the discovery and allow him to further his cause. While discovery was ultimately his plan, if it occurred too soon, all would be lost.
Besides, if he proceeded in this manner, then he was also able to leave a note for posterity. Or, if he were to be completely truthful with himself, as he had vowed to be now that he was finally throwing off the yoke of his oppression, it would make his own victory that much sweeter.
If there was one thing he regretted about life—or at least about the life he finally started to live just shortly before his death—it was that he never was able to see the expression on Voldemort’s face when he realized that his infallible plans for immortality had been discovered and thwarted. Had been discovered and thwarted by the least likely of people, if truth be told.
Perhaps it was a kindness that Regulus Alexander Black died without ever suspecting that his note would not be found by Voldemort. Nay, the Dark Lord would never lay eyes on the message and would forevermore blame an old man and a young boy for the actions which Regulus paid so dearly for.